diff --git a/flask_pydantic/core.py b/flask_pydantic/core.py
index 912415a..4930dad 100644
--- a/flask_pydantic/core.py
+++ b/flask_pydantic/core.py
@@ -182,7 +182,7 @@ def wrapper(*args, **kwargs):
             body_model = body_in_kwargs or body
             if body_model:
                 body_params = get_body_dict(**(get_json_params or {}))
-                if "__root__" in body_model.__fields__:
+                if "__root__" in body_model.model_fields:
                     try:
                         b = body_model(__root__=body_params).__root__
                     except ValidationError as ve: