diff --git a/src/ifcopenshell-python/ifcopenshell/util/unit.py b/src/ifcopenshell-python/ifcopenshell/util/unit.py
index 475aca0f71..45905ca595 100644
--- a/src/ifcopenshell-python/ifcopenshell/util/unit.py
+++ b/src/ifcopenshell-python/ifcopenshell/util/unit.py
@@ -749,10 +749,23 @@ def format_length(
 def is_attr_type(
     content_type: ifcopenshell_wrapper.parameter_type,
     ifc_unit_type_name: str,
+    include_select_types: bool = True
 ) -> Union[ifcopenshell_wrapper.type_declaration, None]:
     cur_decl = content_type
+
+    if include_select_types:
+        if hasattr(cur_decl, "select_list"):
+            for select_item in cur_decl.select_list():
+                if is_attr_type(select_item, ifc_unit_type_name):
+                    return select_item
+
     while hasattr(cur_decl, "declared_type") is True:
         cur_decl = cur_decl.declared_type()
+        if include_select_types:
+            if hasattr(cur_decl, "select_list"):
+                for select_item in cur_decl.select_list():
+                    if is_attr_type(select_item, ifc_unit_type_name):
+                        return select_item
         if hasattr(cur_decl, "name") is False:
             continue
         if cur_decl.name() == ifc_unit_type_name:
@@ -800,6 +813,9 @@ def iter_element_and_attributes_per_type(
             if val is None:
                 continue
 
+            if isinstance(val, ifcopenshell.entity_instance) and not val.is_a(attr_type_name):
+                continue
+
             yield element, attr, val
 
 
@@ -833,8 +849,12 @@ def convert_file_length_units(ifc_file: ifcopenshell.file, target_units: str = "
 
     # Traverse all elements and their nested attributes in the file and convert them
     for element, attr, val in iter_element_and_attributes_per_type(file_patched, "IfcLengthMeasure"):
-        new_value = convert_value(val)
-        setattr(element, attr.name(), new_value)
+        if isinstance(val, ifcopenshell.entity_instance):
+            new_value = convert_value(val.wrappedValue)
+            getattr(element, attr.name()).wrappedValue = new_value
+        else:
+            new_value = convert_value(val)
+            setattr(element, attr.name(), new_value)
 
     file_patched.remove(old_length)
     unit_assignment.Units = tuple([new_length, *unit_assignment.Units])
diff --git a/src/ifcopenshell-python/test/util/test_unit.py b/src/ifcopenshell-python/test/util/test_unit.py
index 59b3c3371b..29a09fea5c 100644
--- a/src/ifcopenshell-python/test/util/test_unit.py
+++ b/src/ifcopenshell-python/test/util/test_unit.py
@@ -82,3 +82,13 @@ class TestFormatLength(test.bootstrap.IFC4):
         assert (
             subject.format_length(25.23, 4, unit_system="imperial", input_unit="inch", output_unit="inch") == '25 1/4"'
         )
+
+
+class TestIsAttrType(test.bootstrap.IFC4):
+    def test_run(self):
+        schema = ifcopenshell.schema_by_name("IFC4")
+        declaration = schema.declaration_by_name("IfcPropertySingleValue")
+        nominal_value = declaration.attribute_by_index(2).type_of_attribute()
+        assert subject.is_attr_type(nominal_value, "IfcValue")
+        assert subject.is_attr_type(nominal_value, "IfcLengthMeasure")
+        assert not subject.is_attr_type(nominal_value, "IfcLengthMeasure", include_select_types=False)