diff --git a/src/caosdb/common/models.py b/src/caosdb/common/models.py
index 9a168a7f83c13d2e2ae1bd7bed857dedc364fa7d..c1240a43c8720a6bbfd11553c3833224592c9de2 100644
--- a/src/caosdb/common/models.py
+++ b/src/caosdb/common/models.py
@@ -2971,7 +2971,9 @@ class Container(list):
                 if is_reference(references.datatype):
                     # add only if it is a reference, not a property
 
-                    if isinstance(references.value, int):
+                    if references.value is None:
+                        continue
+                    elif isinstance(references.value, int):
                         is_being_referenced.add(references.value)
                     elif is_list_datatype(references.datatype):
                         for list_item in references.value: