diff --git a/pydantic_mongo/fields.py b/pydantic_mongo/fields.py
index 1e7fed7..3e6ef2c 100644
--- a/pydantic_mongo/fields.py
+++ b/pydantic_mongo/fields.py
@@ -16,7 +16,7 @@ def __get_pydantic_core_schema__(
             ]
         )
         return core_schema.json_or_python_schema(
-            json_schema=object_id_schema,
+            json_schema=core_schema.str_schema(),
             python_schema=core_schema.union_schema(
                 [core_schema.is_instance_schema(ObjectId), object_id_schema]
             ),