diff --git a/pydantic_extra_types/mongo_object_id.py b/pydantic_extra_types/mongo_object_id.py
index 14336f9..767f7b5 100644
--- a/pydantic_extra_types/mongo_object_id.py
+++ b/pydantic_extra_types/mongo_object_id.py
@@ -57,7 +57,7 @@ def __get_pydantic_core_schema__(cls, _: Any, __: GetCoreSchemaHandler) -> core_
                     ),
                 ]
             ),
-            serialization=core_schema.plain_serializer_function_ser_schema(lambda x: str(x)),
+            serialization=core_schema.plain_serializer_function_ser_schema(lambda x: str(x), when_used='json'),
         )
 
     @classmethod