diff --git a/src/bonsai/bonsai/bim/module/type/operator.py b/src/bonsai/bonsai/bim/module/type/operator.py
index 4a6ce053fc..7b306b1b31 100644
--- a/src/bonsai/bonsai/bim/module/type/operator.py
+++ b/src/bonsai/bonsai/bim/module/type/operator.py
@@ -65,10 +65,28 @@ class AssignType(bpy.types.Operator, tool.Ifc.Operator):
             if active_drawing:
                 active_target_view = tool.Drawing.get_drawing_target_view(active_drawing)
 
+        compatible: list[tuple[bpy.types.Object, ifcopenshell.entity_instance]] = []
+        skipped_classes: set[str] = set()
         for obj in related_objects:
             element = tool.Ifc.get_entity(obj)
             if not element or not element.is_a("IfcObject"):
                 continue
+            if not tool.Type.is_relating_type_compatible(element, relating_type):
+                skipped_classes.add(element.is_a())
+                continue
+            compatible.append((obj, element))
+
+        if skipped_classes:
+            self.report(
+                {"WARNING"},
+                f"Skipped {', '.join(sorted(skipped_classes))}: not a valid occurrence for " f"{relating_type.is_a()}.",
+            )
+
+        if not compatible:
+            self.report({"ERROR"}, f"No selected object can be typed by {relating_type.is_a()}.")
+            return {"CANCELLED"}
+
+        for obj, element in compatible:
             core.assign_type(tool.Ifc, tool.Model, tool.Type, element=element, type=relating_type)
 
             # Switch to the drawing's target view if available
@@ -376,12 +394,22 @@ class DuplicateType(bpy.types.Operator, tool.Ifc.Operator):
         if self.assign_selected_objects:
             selected_objects = tool.Blender.get_selected_objects()
             prefs = tool.Blender.get_addon_preferences()
+            skipped_classes: set[str] = set()
             for selected_obj in selected_objects:
                 selected_element = tool.Ifc.get_entity(selected_obj)
-                if selected_element and selected_element.is_a("IfcObject"):
-                    core.assign_type(tool.Ifc, tool.Model, tool.Type, element=selected_element, type=new)
-                    if prefs.occurrence_name_style == "TYPE":
-                        selected_obj.name = tool.Model.generate_occurrence_name(new, selected_element.is_a())
+                if not selected_element or not selected_element.is_a("IfcObject"):
+                    continue
+                if not tool.Type.is_relating_type_compatible(selected_element, new):
+                    skipped_classes.add(selected_element.is_a())
+                    continue
+                core.assign_type(tool.Ifc, tool.Model, tool.Type, element=selected_element, type=new)
+                if prefs.occurrence_name_style == "TYPE":
+                    selected_obj.name = tool.Model.generate_occurrence_name(new, selected_element.is_a())
+            if skipped_classes:
+                self.report(
+                    {"WARNING"},
+                    f"Skipped {', '.join(sorted(skipped_classes))}: not a valid occurrence for " f"{new.is_a()}.",
+                )
 
         if obj in context.selectable_objects:
             tool.Blender.select_and_activate_single_object(context, new_obj)
diff --git a/src/bonsai/bonsai/core/tool.py b/src/bonsai/bonsai/core/tool.py
index 91003174ad..3aa8d7408f 100644
--- a/src/bonsai/bonsai/core/tool.py
+++ b/src/bonsai/bonsai/core/tool.py
@@ -1208,6 +1208,7 @@ class Type:
     def get_representation_context(cls, representation): pass
     def get_type_occurrences(cls, element_type): pass
     def has_material_usage(cls, element): pass
+    def is_relating_type_compatible(cls, occurrence, relating_type): pass
     def record_material_usage_attributes(cls, element): pass
     def restore_material_usage_attributes(cls, element, usage_attributes): pass
     def run_geometry_add_representation(cls, obj=None, context=None, ifc_representation_class=None, profile_set_usage=None): pass
diff --git a/src/bonsai/bonsai/tool/type.py b/src/bonsai/bonsai/tool/type.py
index 882c4b4618..f2617ad060 100644
--- a/src/bonsai/bonsai/tool/type.py
+++ b/src/bonsai/bonsai/tool/type.py
@@ -24,6 +24,7 @@ import bpy
 import ifcopenshell
 import ifcopenshell.util.element
 import ifcopenshell.util.representation
+import ifcopenshell.util.type
 
 import bonsai.core.geometry
 import bonsai.core.tool
@@ -96,6 +97,18 @@ class Type(bonsai.core.tool.Type):
     def get_type_occurrences(cls, element_type: ifcopenshell.entity_instance) -> list[ifcopenshell.entity_instance]:
         return ifcopenshell.util.element.get_types(element_type)
 
+    @classmethod
+    def is_relating_type_compatible(
+        cls,
+        occurrence: ifcopenshell.entity_instance,
+        relating_type: ifcopenshell.entity_instance,
+    ) -> bool:
+        # IFC's EXPRESS schema has no WHERE rule pairing IfcRelDefinesByType's
+        # RelatingType / RelatedObjects classes; the one-to-one class pairing
+        # is a buildingSMART implementer agreement, not file-validation.
+        schema = occurrence.file.schema
+        return relating_type.is_a() in ifcopenshell.util.type.get_applicable_types(occurrence.is_a(), schema=schema)
+
     @classmethod
     def has_material_usage(cls, element: ifcopenshell.entity_instance) -> bool:
         material = ifcopenshell.util.element.get_material(element)
diff --git a/src/bonsai/test/bim/module/type/__init__.py b/src/bonsai/test/bim/module/type/__init__.py
new file mode 100644
index 0000000000..e69de29bb2
diff --git a/src/bonsai/test/bim/module/type/test_assign_type_forward_compat.py b/src/bonsai/test/bim/module/type/test_assign_type_forward_compat.py
new file mode 100644
index 0000000000..106c517565
--- /dev/null
+++ b/src/bonsai/test/bim/module/type/test_assign_type_forward_compat.py
@@ -0,0 +1,104 @@
+# Bonsai - OpenBIM Blender Add-on
+# Copyright (C) 2026
+#
+# This file is part of Bonsai.
+#
+# Bonsai is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# Bonsai is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with Bonsai.  If not, see <http://www.gnu.org/licenses/>.
+#
+# This file was generated with the assistance of an AI coding tool.
+
+"""Forward-compat AST contracts for the class-mismatched-type-assignment guard.
+
+Two structural invariants that no behavioural test can pin on its own:
+
+1. ``ifcopenshell.api.type.assign_type`` MUST reference
+   ``ifcopenshell.util.type.get_applicable_entities`` (or
+   ``get_applicable_types``) — the schema-aware applicability lookup that
+   produces the canonical class-pairing whitelist. A drift here means the
+   API stops rejecting class-mismatched pairs.
+
+2. ``bonsai.bim.module.type.operator`` MUST reference
+   ``tool.Type.is_relating_type_compatible`` — the single source of truth
+   for partition / WARNING / CANCELLED behaviour in the Bonsai operator
+   layer. A drift here re-opens the fan-out hole that silently writes
+   schema-corrupt typings into the selection when one (active) object's
+   class drove the picker but other selected objects don't match.
+"""
+
+import ast
+from pathlib import Path
+
+import pytest
+
+pytestmark = pytest.mark.type
+
+
+BONSAI_ROOT = Path(__file__).resolve().parents[4] / "bonsai"
+IFCOPENSHELL_API_ASSIGN_TYPE = (
+    Path(__file__).resolve().parents[5] / "ifcopenshell-python" / "ifcopenshell" / "api" / "type" / "assign_type.py"
+)
+BONSAI_TYPE_OPERATOR = BONSAI_ROOT / "bim" / "module" / "type" / "operator.py"
+
+
+def _attribute_chain(node: ast.AST) -> str:
+    """Render an ``ast.Attribute``/``ast.Name`` chain as a dotted string,
+    e.g. ``tool.Type.is_relating_type_compatible``. Returns ``""`` if the
+    chain bottoms out on something other than a Name (e.g. a subscript)."""
+    parts: list[str] = []
+    while isinstance(node, ast.Attribute):
+        parts.append(node.attr)
+        node = node.value
+    if isinstance(node, ast.Name):
+        parts.append(node.id)
+        return ".".join(reversed(parts))
+    return ""
+
+
+def _all_attribute_chains(tree: ast.Module) -> set[str]:
+    chains: set[str] = set()
+    for node in ast.walk(tree):
+        if isinstance(node, ast.Attribute):
+            chain = _attribute_chain(node)
+            if chain:
+                chains.add(chain)
+    return chains
+
+
+def test_api_assign_type_calls_applicability_lookup() -> None:
+    """Pin Layer B: ``ifcopenshell.api.type.assign_type`` references
+    ``ifcopenshell.util.type.get_applicable_entities`` (the source of truth
+    for which occurrence classes a given type class may type)."""
+    tree = ast.parse(IFCOPENSHELL_API_ASSIGN_TYPE.read_text(encoding="utf-8"))
+    chains = _all_attribute_chains(tree)
+    sentinel = "ifcopenshell.util.type.get_applicable_entities"
+    assert sentinel in chains, (
+        f"{IFCOPENSHELL_API_ASSIGN_TYPE.name} no longer references {sentinel}. "
+        "The API-layer guard against class-mismatched type assignment is gone."
+    )
+
+
+def test_bonsai_type_operator_module_references_compatibility_helper() -> None:
+    """Pin Layer C: the Bonsai type operator module references
+    ``tool.Type.is_relating_type_compatible``. Every operator in this file
+    that fans assign_type calls across a multi-selection must filter
+    through this helper to avoid writing mismatched typings on objects the
+    panel picker didn't validate."""
+    tree = ast.parse(BONSAI_TYPE_OPERATOR.read_text(encoding="utf-8"))
+    chains = _all_attribute_chains(tree)
+    sentinel = "tool.Type.is_relating_type_compatible"
+    assert sentinel in chains, (
+        f"{BONSAI_TYPE_OPERATOR.name} no longer references {sentinel}. "
+        "Operator-layer partition that prevents schema-illegal type "
+        "assignment across multi-selection has been removed."
+    )
diff --git a/src/bonsai/test/bim/module/type/test_assign_type_partition.py b/src/bonsai/test/bim/module/type/test_assign_type_partition.py
new file mode 100644
index 0000000000..231f215135
--- /dev/null
+++ b/src/bonsai/test/bim/module/type/test_assign_type_partition.py
@@ -0,0 +1,176 @@
+# Bonsai - OpenBIM Blender Add-on
+# Copyright (C) 2026
+#
+# This file is part of Bonsai.
+#
+# Bonsai is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# Bonsai is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU Lesser General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with Bonsai.  If not, see <http://www.gnu.org/licenses/>.
+#
+# This file was generated with the assistance of an AI coding tool.
+
+"""Contract test for BIM_OT_assign_type's per-object class-compatibility
+partition.
+
+When the user multi-selects mixed classes (e.g. a wall + a door), the panel
+picker filters the class dropdown by the active object's class only.
+Historically the operator then fanned out across the whole selection without
+re-checking each occurrence, producing schema-corrupt IFC files (IfcDoor
+typed by IfcWallType). The partition added in this change must:
+
+1. Assign the type only to compatible occurrences.
+2. Surface skipped classes through ``self.report({'WARNING'}, ...)``.
+3. ``return {'CANCELLED'}`` and emit an ERROR when nothing in the selection
+   is compatible — no mutation must reach ``core.assign_type``.
+"""
+
+from unittest import mock
+
+import pytest
+
+pytestmark = pytest.mark.type
+
+
+@pytest.fixture(autouse=True)
+def _require_real_bpy():
+    import types as _types
+
+    import bpy
+
+    if not isinstance(bpy, _types.ModuleType) or hasattr(bpy, "_mock_name"):
+        pytest.skip("requires real Blender (bpy is mocked or absent)")
+
+
+@pytest.fixture
+def fresh_ifc():
+    import ifcopenshell
+
+    from bonsai.bim.ifc import IfcStore
+
+    previous = IfcStore.file
+    IfcStore.file = ifcopenshell.file(schema="IFC4")
+    try:
+        yield IfcStore.file
+    finally:
+        IfcStore.file = previous
+
+
+def _make_object(name, element):
+    """Build a real bpy.types.Object linked to an IFC entity via
+    tool.Ifc.link, so tool.Ifc.get_entity(obj) resolves correctly."""
+    import bpy
+
+    import bonsai.tool as tool
+
+    obj = bpy.data.objects.new(name, None)
+    tool.Ifc.link(element, obj)
+    return obj
+
+
+def _execute_assign(op, context):
+    """Drive ``AssignType._execute`` directly. Bypasses the framework's
+    transaction wrapping so a unit test can observe the partition without
+    setting up the full Blender harness."""
+    return op._execute(context)
+
+
+@pytest.fixture
+def neutralised_side_effects():
+    """Patch the helpers ``AssignType._execute`` calls outside the partition
+    logic (addon prefs, drawing context lookup, drawing target-view branch),
+    so the test asserts only the partition / report / return-code contract."""
+    with mock.patch("bonsai.bim.module.type.operator.tool.Blender.get_addon_preferences") as prefs:
+        prefs.return_value = mock.Mock(occurrence_name_style="OCCURRENCE")
+        yield
+
+
+def _build_context_with_no_active_drawing():
+    """Return a Mock ``context`` whose ``scene.DocProperties.active_drawing_id``
+    is 0, skipping the drawing-target-view block in ``_execute``."""
+    context = mock.Mock()
+    context.scene.DocProperties.active_drawing_id = 0
+    return context
+
+
+def _fake_operator_with_report():
+    """Build a Mock that satisfies the attribute reads ``AssignType._execute``
+    makes on ``self`` (``relating_type``, ``related_object``, ``report``)."""
+    op = mock.MagicMock()
+    op.relating_type = 0
+    op.related_object = ""
+    op.report = mock.Mock()
+    return op
+
+
+def test_mixed_selection_assigns_only_compatible_objects(fresh_ifc, neutralised_side_effects):
+    """Wall + door selected, IfcWallType picked: wall gets typed, door is
+    skipped with a WARNING, and the operator returns success.
+
+    ``core.assign_type`` is mocked: it would otherwise run the
+    representation-switch and material plumbing on stub Blender objects.
+    The contract under test is the partition / report logic, not the
+    downstream representation pipeline."""
+    import ifcopenshell.api.root
+
+    from bonsai.bim.module.type.operator import AssignType
+
+    wall_elem = ifcopenshell.api.root.create_entity(fresh_ifc, ifc_class="IfcWall")
+    door_elem = ifcopenshell.api.root.create_entity(fresh_ifc, ifc_class="IfcDoor")
+    wall_type = ifcopenshell.api.root.create_entity(fresh_ifc, ifc_class="IfcWallType")
+
+    wall_obj = _make_object("Wall", wall_elem)
+    door_obj = _make_object("Door", door_elem)
+
+    op = _fake_operator_with_report()
+    op.relating_type = wall_type.id()
+
+    with mock.patch(
+        "bonsai.bim.module.type.operator.tool.Blender.get_selected_objects", return_value=[wall_obj, door_obj]
+    ), mock.patch("bonsai.bim.module.type.operator.core.assign_type") as mock_assign:
+        result = AssignType._execute(op, _build_context_with_no_active_drawing())
+
+    assert result != {"CANCELLED"}, "operator must succeed when at least one object is compatible"
+    typed_elements = {call.kwargs["element"] for call in mock_assign.call_args_list}
+    assert typed_elements == {
+        wall_elem
+    }, f"only the compatible wall element should reach core.assign_type, got {typed_elements}"
+
+    warning_calls = [c for c in op.report.call_args_list if c.args[0] == {"WARNING"}]
+    assert warning_calls, "skipped occurrence class must surface as a WARNING"
+    assert any("IfcDoor" in c.args[1] for c in warning_calls)
+
+
+def test_all_incompatible_selection_returns_cancelled_without_mutation(fresh_ifc, neutralised_side_effects):
+    """Door alone selected, IfcWallType picked: nothing to assign. Operator
+    must return CANCELLED, emit an ERROR, and never call core.assign_type."""
+    import ifcopenshell.api.root
+    import ifcopenshell.util.element
+
+    from bonsai.bim.module.type.operator import AssignType
+
+    door_elem = ifcopenshell.api.root.create_entity(fresh_ifc, ifc_class="IfcDoor")
+    wall_type = ifcopenshell.api.root.create_entity(fresh_ifc, ifc_class="IfcWallType")
+    door_obj = _make_object("Door", door_elem)
+
+    op = _fake_operator_with_report()
+    op.relating_type = wall_type.id()
+
+    with mock.patch(
+        "bonsai.bim.module.type.operator.tool.Blender.get_selected_objects", return_value=[door_obj]
+    ), mock.patch("bonsai.bim.module.type.operator.core.assign_type") as mock_assign:
+        result = AssignType._execute(op, _build_context_with_no_active_drawing())
+
+    assert result == {"CANCELLED"}
+    assert mock_assign.call_count == 0
+    error_calls = [c for c in op.report.call_args_list if c.args[0] == {"ERROR"}]
+    assert error_calls, "all-incompatible selection must surface as an ERROR"
+    assert ifcopenshell.util.element.get_type(door_elem) is None
diff --git a/src/bonsai/test/tool/test_type.py b/src/bonsai/test/tool/test_type.py
index 0d91b1f4f0..7ff096e0e8 100644
--- a/src/bonsai/test/tool/test_type.py
+++ b/src/bonsai/test/tool/test_type.py
@@ -172,6 +172,48 @@ class TestHasMaterialUsage(NewFile):
         assert subject.has_material_usage(element) is True
 
 
+class TestIsRelatingTypeCompatible(NewFile):
+    def test_matched_pair_ifc4(self):
+        ifc = ifcopenshell.file()
+        door = ifcopenshell.api.root.create_entity(ifc, ifc_class="IfcDoor")
+        door_type = ifcopenshell.api.root.create_entity(ifc, ifc_class="IfcDoorType")
+        assert subject.is_relating_type_compatible(door, door_type) is True
+
+    def test_mismatched_pair_ifc4(self):
+        ifc = ifcopenshell.file()
+        door = ifcopenshell.api.root.create_entity(ifc, ifc_class="IfcDoor")
+        wall_type = ifcopenshell.api.root.create_entity(ifc, ifc_class="IfcWallType")
+        assert subject.is_relating_type_compatible(door, wall_type) is False
+
+    def test_legacy_style_pairing_allowed_in_ifc4(self):
+        ifc = ifcopenshell.file()
+        door = ifcopenshell.api.root.create_entity(ifc, ifc_class="IfcDoor")
+        door_style = ifcopenshell.api.root.create_entity(ifc, ifc_class="IfcDoorStyle")
+        assert subject.is_relating_type_compatible(door, door_style) is True
+
+    def test_legacy_style_pairing_refused_in_ifc4x3(self):
+        ifc = ifcopenshell.file(schema="IFC4X3")
+        door = ifcopenshell.api.root.create_entity(ifc, ifc_class="IfcDoor")
+        try:
+            door_style = ifcopenshell.api.root.create_entity(ifc, ifc_class="IfcDoorStyle")
+        except Exception:
+            # IfcDoorStyle was removed in IFC4X3 — exclusion holds trivially.
+            return
+        assert subject.is_relating_type_compatible(door, door_style) is False
+
+    def test_untypable_occurrence_returns_false(self):
+        ifc = ifcopenshell.file()
+        opening = ifcopenshell.api.root.create_entity(ifc, ifc_class="IfcOpeningElement")
+        any_type = ifcopenshell.api.root.create_entity(ifc, ifc_class="IfcDoorType")
+        assert subject.is_relating_type_compatible(opening, any_type) is False
+
+    def test_proxy_type_pairing(self):
+        ifc = ifcopenshell.file()
+        proxy = ifcopenshell.api.root.create_entity(ifc, ifc_class="IfcBuildingElementProxy")
+        proxy_type = ifcopenshell.api.root.create_entity(ifc, ifc_class="IfcBuildingElementProxyType")
+        assert subject.is_relating_type_compatible(proxy, proxy_type) is True
+
+
 class TestRunGeometryAddRepresentation(NewFile):
     def test_nothing(self):
         pass
diff --git a/src/ifcopenshell-python/ifcopenshell/api/type/assign_type.py b/src/ifcopenshell-python/ifcopenshell/api/type/assign_type.py
index 2c7abcd169..9dd01ee97e 100644
--- a/src/ifcopenshell-python/ifcopenshell/api/type/assign_type.py
+++ b/src/ifcopenshell-python/ifcopenshell/api/type/assign_type.py
@@ -24,6 +24,7 @@ import ifcopenshell.api.owner
 import ifcopenshell.api.type
 import ifcopenshell.guid
 import ifcopenshell.util.element
+import ifcopenshell.util.type
 
 
 def assign_type(
@@ -189,6 +190,20 @@ class Usecase:
         if not related_objects:
             return
 
+        # The EXPRESS schema has no WHERE rule pairing RelatingType /
+        # RelatedObjects classes; the canonical class pairing per schema
+        # is a buildingSMART implementer agreement, enforced here.
+        allowed_occurrences = ifcopenshell.util.type.get_applicable_entities(
+            relating_type.is_a(), schema=self.file.schema
+        )
+        mismatched_classes = sorted({o.is_a() for o in related_objects if o.is_a() not in allowed_occurrences})
+        if mismatched_classes:
+            raise TypeError(
+                f"{relating_type.is_a()} cannot type {', '.join(mismatched_classes)} "
+                f"in schema {self.file.schema} (allowed occurrence classes: "
+                f"{allowed_occurrences or '<none>'})"
+            )
+
         ifc2x3 = self.file.schema == "IFC2X3"
         related_objects_set = set(related_objects)
         if ifc2x3:
diff --git a/src/ifcopenshell-python/test/api/type/test_assign_type.py b/src/ifcopenshell-python/test/api/type/test_assign_type.py
index 009d4efedf..1125fd3888 100644
--- a/src/ifcopenshell-python/test/api/type/test_assign_type.py
+++ b/src/ifcopenshell-python/test/api/type/test_assign_type.py
@@ -183,6 +183,39 @@ class TestAssignType(test.bootstrap.IFC4):
             assert element.PredefinedType == "USERDEFINED"
         assert element.ObjectType == "Test"
 
+    def test_class_mismatched_pair_raises(self):
+        door = ifcopenshell.api.root.create_entity(self.file, ifc_class="IfcDoor")
+        wall_type = ifcopenshell.api.root.create_entity(self.file, ifc_class="IfcWallType")
+        with pytest.raises(TypeError, match=r"IfcWallType cannot type IfcDoor"):
+            ifcopenshell.api.type.assign_type(self.file, related_objects=[door], relating_type=wall_type)
+        assert ifcopenshell.util.element.get_type(door) is None
+
+    def test_class_mismatched_pair_does_not_mutate(self):
+        door = ifcopenshell.api.root.create_entity(self.file, ifc_class="IfcDoor")
+        wall_type = ifcopenshell.api.root.create_entity(self.file, ifc_class="IfcWallType")
+        rels_before = self.file.by_type("IfcRelDefinesByType")
+        with pytest.raises(TypeError):
+            ifcopenshell.api.type.assign_type(self.file, related_objects=[door], relating_type=wall_type)
+        rels_after = self.file.by_type("IfcRelDefinesByType")
+        assert rels_after == rels_before
+
+    def test_partial_mismatch_in_selection_rejects_whole_call(self):
+        door = ifcopenshell.api.root.create_entity(self.file, ifc_class="IfcDoor")
+        wall = ifcopenshell.api.root.create_entity(self.file, ifc_class="IfcWall")
+        wall_type = ifcopenshell.api.root.create_entity(self.file, ifc_class="IfcWallType")
+        with pytest.raises(TypeError):
+            ifcopenshell.api.type.assign_type(self.file, related_objects=[door, wall], relating_type=wall_type)
+        # The good occurrence must NOT have been typed — partial mutation is the
+        # bug class this guard exists to prevent.
+        assert ifcopenshell.util.element.get_type(wall) is None
+        assert ifcopenshell.util.element.get_type(door) is None
+
+    def test_untypable_occurrence_rejected(self):
+        opening = ifcopenshell.api.root.create_entity(self.file, ifc_class="IfcOpeningElement")
+        any_type = ifcopenshell.api.root.create_entity(self.file, ifc_class="IfcWallType")
+        with pytest.raises(TypeError):
+            ifcopenshell.api.type.assign_type(self.file, related_objects=[opening], relating_type=any_type)
+
 
 class TestAssignTypeIFC2X3(test.bootstrap.IFC2X3, TestAssignType):
     pass