diff --git a/src/ifcopenshell-python/ifcopenshell/entity_instance.py b/src/ifcopenshell-python/ifcopenshell/entity_instance.py
index badbaed3d4..675d35420f 100644
--- a/src/ifcopenshell-python/ifcopenshell/entity_instance.py
+++ b/src/ifcopenshell-python/ifcopenshell/entity_instance.py
@@ -108,22 +108,33 @@ class entity_instance(object):
         return entity_instance.wrap_value(self.wrapped_data.get_argument(key))
 
     def __setitem__(self, idx, value):
+        attr_type = real_attr_type = self.attribute_type(idx).title().replace(' ', '')
+        real_attr_type = real_attr_type.replace('Derived', 'None')
+        attr_type = attr_type.replace('Binary', 'String')
+        attr_type = attr_type.replace('Enumeration', 'String')
+        
         if value is None:
-            self.wrapped_data.setArgumentAsNull(idx)
+            if attr_type != "Derived":
+                self.wrapped_data.setArgumentAsNull(idx)
         else:
-            attr_type = real_attr_type = self.attribute_type(idx).title().replace(' ', '')
-            attr_type = attr_type.replace('Binary', 'String')
-            attr_type = attr_type.replace('Enumeration', 'String')
-            try:
-                if isinstance(value, unicode):
-                    value = value.encode("utf-8")
-            except BaseException:
-                pass
-            try:
-                getattr(self.wrapped_data, "setArgumentAs%s" % attr_type)(idx, entity_instance.unwrap_value(value))
-            except BaseException:
+            valid = attr_type != "Derived"
+            if valid:                 
+                try:
+                    if isinstance(value, unicode):
+                        value = value.encode("utf-8")
+                except BaseException:
+                    pass
+                    
+                try:
+                    if attr_type != "Derived":
+                        getattr(self.wrapped_data, "setArgumentAs%s" % attr_type)(idx, entity_instance.unwrap_value(value))
+                except BaseException as e:
+                    valid = False
+
+            if not valid:
                 raise ValueError("Expected %s for attribute %s.%s, got %r" % (
                     real_attr_type, self.is_a(), self.attribute_name(idx), value))
+
         return value
 
     def __len__(self):