diff --git a/rsconcept/backend/apps/rsform/serializers/data_access.py b/rsconcept/backend/apps/rsform/serializers/data_access.py
index 5d6641bd..c2c63d6b 100644
--- a/rsconcept/backend/apps/rsform/serializers/data_access.py
+++ b/rsconcept/backend/apps/rsform/serializers/data_access.py
@@ -179,7 +179,7 @@ class RSFormSerializer(serializers.ModelSerializer):
                 cst.delete()
             else:
                 cst_data = next(x for x in items if x['id'] == cst.pk)
-                cst_data['schema'] = self.instance.pk
+                cst_data['schema'] = cast(LibraryItem, self.instance).pk
                 new_cst = CstBaseSerializer(data=cst_data)
                 new_cst.is_valid(raise_exception=True)
                 new_cst.validated_data['order'] = ids.index(cst.pk)
@@ -194,7 +194,7 @@ class RSFormSerializer(serializers.ModelSerializer):
                 cst = schema.insert_new(cst_data['alias'])
                 old_id = cst_data['id']
                 cst_data['id'] = cst.pk
-                cst_data['schema'] = self.instance.pk
+                cst_data['schema'] = cast(LibraryItem, self.instance).pk
                 new_cst = CstBaseSerializer(data=cst_data)
                 new_cst.is_valid(raise_exception=True)
                 new_cst.validated_data['order'] = ids.index(old_id)