diff --git a/src/bonsai/bonsai/core/material.py b/src/bonsai/bonsai/core/material.py
index 96c624de64..b70bed7bff 100644
--- a/src/bonsai/bonsai/core/material.py
+++ b/src/bonsai/bonsai/core/material.py
@@ -142,8 +142,35 @@ def assign_material(
         else:
             element_material_type = material_type
 
-        ifc.run("material.assign_material", products=[element], type=element_material_type, material=material)
-        assigned_material = material_tool.get_material(element)
+        # TODO: this whole dance is a stopgap and wants rewriting.
+        #
+        # material.assign_material creates material sets with no items in them,
+        # ignoring the material it was handed -- an IfcMaterialLayerSet with no
+        # MaterialLayers is not valid IFC, since the list is mandatory and
+        # [1:?]. So we repair it below, after the fact. Worse, the API rejects a
+        # plain IfcMaterial outright when asked for a usage, which is exactly
+        # what the Object Materials dropdown gives us, so we cannot even pass it
+        # on and have to let the API invent an empty set and then fill it in.
+        #
+        # The fix is for assign_material to build the set around the material it
+        # is given, rather than leaving an invalid one behind for its callers to
+        # patch up. That is a wider change than it looks: add_material_set has
+        # the same behaviour, and the create-empty-then-add-items idiom is
+        # spread through the API's own docstrings, examples and tests. Until
+        # that is untangled, keep the repair here where it is at least visible.
+
+        # Only a usage refuses a plain IfcMaterial; every other type still wants
+        # it, and IfcMaterial and IfcMaterialList cannot be created without it.
+        pass_material = material_tool.is_a_material_set(material) or not element_material_type.endswith("Usage")
+        ifc.run(
+            "material.assign_material",
+            products=[element],
+            type=element_material_type,
+            material=material if pass_material else None,
+        )
+        # A usage points at the set rather than being one, and it is the set
+        # that needs an item adding to it below.
+        assigned_material = material_tool.get_material(element, should_skip_usage=True)
         assert assigned_material  # Type checker.
 
         if material_tool.is_a_material_set(material):
diff --git a/src/bonsai/bonsai/core/tool.py b/src/bonsai/bonsai/core/tool.py
index 6280e78309..7ba5e715db 100644
--- a/src/bonsai/bonsai/core/tool.py
+++ b/src/bonsai/bonsai/core/tool.py
@@ -651,7 +651,7 @@ class Material:
     def get_default_material(cls): pass
     def get_elements_by_material(cls, material): pass
     def get_material_attributes(cls): pass
-    def get_material(cls, element, should_inherit: bool = False): pass
+    def get_material(cls, element, should_inherit: bool = False, should_skip_usage: bool = False): pass
     def get_object_ui_active_material(cls): pass
     def get_object_ui_material_type(cls): pass
     def get_style(cls, material): pass
diff --git a/src/bonsai/bonsai/tool/material.py b/src/bonsai/bonsai/tool/material.py
index 300df8d4ce..695759227c 100644
--- a/src/bonsai/bonsai/tool/material.py
+++ b/src/bonsai/bonsai/tool/material.py
@@ -220,9 +220,14 @@ class Material(bonsai.core.tool.Material):
 
     @classmethod
     def get_material(
-        cls, element: ifcopenshell.entity_instance, should_inherit: bool = False
+        cls,
+        element: ifcopenshell.entity_instance,
+        should_inherit: bool = False,
+        should_skip_usage: bool = False,
     ) -> Union[ifcopenshell.entity_instance, None]:
-        return ifcopenshell.util.element.get_material(element, should_inherit=should_inherit)
+        return ifcopenshell.util.element.get_material(
+            element, should_inherit=should_inherit, should_skip_usage=should_skip_usage
+        )
 
     @classmethod
     def is_a_material_set(cls, material: ifcopenshell.entity_instance) -> bool:
diff --git a/src/bonsai/test/bim/test_feature.py b/src/bonsai/test/bim/test_feature.py
index 34150fbe98..88697b7c92 100644
--- a/src/bonsai/test/bim/test_feature.py
+++ b/src/bonsai/test/bim/test_feature.py
@@ -1000,6 +1000,7 @@ def i_click_button_and_expect_error_error_msg(button, error_msg):
 
 @given(parsers.parse('I evaluate expression "{expression}"'))
 @when(parsers.parse('I evaluate expression "{expression}"'))
+@then(parsers.parse('I evaluate expression "{expression}"'))
 def i_evaluate_expression(expression):
     expression = replace_variables(expression)
     exec(expression)