diff --git a/src/caosdb/common/models.py b/src/caosdb/common/models.py
index 417d0556ed8c0a40d925dd96e2ce4f7240053085..f9d4f2b07b075eda7d50394c4f95a113f815a5bf 100644
--- a/src/caosdb/common/models.py
+++ b/src/caosdb/common/models.py
@@ -39,6 +39,7 @@ from tempfile import NamedTemporaryFile
 from warnings import warn
 
 from lxml import etree
+from .datatype import is_reference
 from caosdb.common.utils import uuid, xml2str
 from caosdb.configuration import get_config
 from caosdb.connection.connection import get_connection