diff --git a/src/caosdb/common/datatype.py b/src/caosdb/common/datatype.py
index eb8c1e4e0088f1924940a104ec3916b9d5d40f99..b6e1f7ac8c1307d5e4a2957a802f9a35ad77e1b7 100644
--- a/src/caosdb/common/datatype.py
+++ b/src/caosdb/common/datatype.py
@@ -66,6 +66,10 @@ def is_reference(datatype):
     RecordTypes
     """
 
+    if datatype is None:
+        raise ValueError("Cannot decide whether datatype is reference if None"
+                         " is supplied")
+
     if datatype in [DOUBLE, BOOLEAN, INTEGER, TEXT, DATETIME]:
         return False
     elif is_list_datatype(datatype):