--- a/wtforms/ext/sqlalchemy/fields.py
+++ b/wtforms/ext/sqlalchemy/fields.py
@@ -186,5 +186,5 @@ class QuerySelectMultipleField(QuerySele
 
 
 def get_pk_from_identity(obj):
-    cls, key = identity_key(instance=obj)
+    key = identity_key(instance=obj)[1]
     return ':'.join(text_type(x) for x in key)
