diff --git a/src/ifcopenshell-python/ifcopenshell/sql.py b/src/ifcopenshell-python/ifcopenshell/sql.py
index 0867a9838e..29c08f3a44 100644
--- a/src/ifcopenshell-python/ifcopenshell/sql.py
+++ b/src/ifcopenshell-python/ifcopenshell/sql.py
@@ -380,9 +380,9 @@ class sqlite_entity:
         INVALID, FORWARD, INVERSE, DERIVED = range(4)
         attr_cat = self.wrapped_data.get_attribute_category(name)
         if attr_cat == DERIVED:
-            # Derived attributes aren't stored or computed for SQLite-linked
-            # files, so callers must treat None as "not available" here.
-            return None
+            raise RuntimeError(
+                f"Derived attributes (e.g. '{name}') are not supported for {type(self).__name__} entities."
+            )
         if attr_cat == FORWARD:
             if self.sqlite_wrapper.attribute_cache:
                 return self.sqlite_wrapper.attribute_cache[name]
diff --git a/src/ifcopenshell-python/ifcopenshell/stream.py b/src/ifcopenshell-python/ifcopenshell/stream.py
index 2898f28c8a..8eb6afbcac 100644
--- a/src/ifcopenshell-python/ifcopenshell/stream.py
+++ b/src/ifcopenshell-python/ifcopenshell/stream.py
@@ -360,8 +360,12 @@ try:
             assert False, "Not supported during streaming."
 
         def __getattr__(self, name: str) -> Any:
-            INVALID, FORWARD, INVERSE = range(3)
+            INVALID, FORWARD, INVERSE, DERIVED = range(4)
             attr_cat = self.wrapped_data.get_attribute_category(name)
+            if attr_cat == DERIVED:
+                raise RuntimeError(
+                    f"Derived attributes (e.g. '{name}') are not supported for {type(self).__name__} entities."
+                )
             if attr_cat == FORWARD:
                 if self.stream_wrapper.attribute_cache:
                     return self.stream_wrapper.attribute_cache[name]
diff --git a/src/ifcopenshell-python/test/test_sql.py b/src/ifcopenshell-python/test/test_sql.py
index 39cf4c2583..3f3f39aa8f 100644
--- a/src/ifcopenshell-python/test/test_sql.py
+++ b/src/ifcopenshell-python/test/test_sql.py
@@ -19,6 +19,7 @@
 import tempfile
 from pathlib import Path
 
+import pytest
 from ifcpatch.recipes import Ifc2Sql
 
 import ifcopenshell
@@ -63,6 +64,12 @@ class TestEntity:
         assert (element := ifc_sqlite.by_id(1))
         assert element.Name == "My Project"
 
+    def test_getattr_derive(self):
+        ifc_sqlite = get_ifc_sqlite()
+        assert (units := ifc_sqlite.by_type("IfcSIUnit"))
+        with pytest.raises(RuntimeError):
+            units[0].Dimensions
+
     def test_setattr(self):
         ifc_sqlite = get_ifc_sqlite()
         assert (element := ifc_sqlite.by_id(1))
diff --git a/src/ifcopenshell-python/test/test_stream.py b/src/ifcopenshell-python/test/test_stream.py
index 754fcc4527..b630054556 100644
--- a/src/ifcopenshell-python/test/test_stream.py
+++ b/src/ifcopenshell-python/test/test_stream.py
@@ -18,6 +18,8 @@
 
 from pathlib import Path
 
+import pytest
+
 import ifcopenshell
 
 TEST_FILE = Path(__file__).parent / "files" / "basic.ifc"
@@ -48,3 +50,10 @@ class TestEntity:
         stream_file = ifcopenshell.open(TEST_FILE, should_stream=True)
         assert (element := stream_file.by_id(1))
         assert element.Name == "My Project"
+
+    def test_getattr_derive(self):
+        stream_file: ifcopenshell.stream
+        stream_file = ifcopenshell.open(TEST_FILE, should_stream=True)
+        assert (units := stream_file.by_type("IfcSIUnit"))
+        with pytest.raises(RuntimeError):
+            units[0].Dimensions
diff --git a/src/ifcopenshell-python/test/util/test_unit.py b/src/ifcopenshell-python/test/util/test_unit.py
index c1e902583f..aaca7eb600 100644
--- a/src/ifcopenshell-python/test/util/test_unit.py
+++ b/src/ifcopenshell-python/test/util/test_unit.py
@@ -329,11 +329,9 @@ class TestCalculateUnitScaleOnLinkedFile(test.bootstrap.IFC4):
     def test_run(self):
         # Regression test: IfcSIUnit.Dimensions is a schema-*derived*
         # attribute that isn't computed for SQLite-linked files (used for
-        # Bonsai's "linked project" large-model workflow), so it returns None
-        # there instead of an IfcDimensionalExponents entity. calculate_unit_scale()
-        # used to access unit.Dimensions.LengthExponent unconditionally for
-        # every IfcSIUnit, which crashed project loading for any linked file.
-        # See the PR discussion for a standalone reproduction script.
+        # Bonsai's "linked project" large-model workflow) and raises there
+        # instead of returning an IfcDimensionalExponents entity.
+        # Test ensures `calculate_unit_scale` doesn't rely on derived attribute computation.
         ifcopenshell.api.root.create_entity(self.file, ifc_class="IfcProject")
         length = ifcopenshell.api.unit.add_si_unit(self.file, unit_type="LENGTHUNIT")
         ifcopenshell.api.unit.assign_unit(self.file, units=[length])
@@ -345,7 +343,6 @@ class TestCalculateUnitScaleOnLinkedFile(test.bootstrap.IFC4):
 
         try:
             linked_file = ifcopenshell.open(str(tmp_file))
-            assert linked_file.by_type("IfcSIUnit")[0].Dimensions is None
             assert subject.calculate_unit_scale(linked_file, "LENGTHUNIT") == 1.0
         finally:
             if isinstance(linked_file, ifcopenshell.sqlite):