diff --git a/flask_admin/contrib/sqla/fields.py b/flask_admin/contrib/sqla/fields.py
index 914607e42..88141e0ca 100644
--- a/flask_admin/contrib/sqla/fields.py
+++ b/flask_admin/contrib/sqla/fields.py
@@ -276,7 +276,7 @@ def populate_obj(self, obj, name):
 
         # Handle request data
         for field in self.entries:
-            field_id = field.get_pk()
+            field_id = str(field.get_pk())
 
             is_created = field_id not in pk_map
             if not is_created: