diff --git a/wtforms_sqlalchemy/fields.py b/wtforms_sqlalchemy/fields.py
index 286dada..35a0dc3 100644
--- a/wtforms_sqlalchemy/fields.py
+++ b/wtforms_sqlalchemy/fields.py
@@ -186,5 +186,5 @@ def pre_validate(self, form):
 
 
 def get_pk_from_identity(obj):
-    cls, key = identity_key(instance=obj)
+    cls, key = identity_key(instance=obj)[:2]
     return ':'.join(text_type(x) for x in key)