diff --git a/flask_security/datastore.py b/flask_security/datastore.py
index b689c076..c3398145 100644
--- a/flask_security/datastore.py
+++ b/flask_security/datastore.py
@@ -118,7 +118,8 @@ def __init__(self, user_model, role_model):
 
     def _prepare_role_modify_args(self, user, role):
         if isinstance(user, string_types):
-            user = self.find_user(email=user)
+            identity_attr = get_identity_attributes()[0]
+            user = self.find_user(**{identity_attr: user})
         if isinstance(role, string_types):
             role = self.find_role(role)
         return user, role