From b19ef4b70e99fd6cd3b8813fe1523b41eb9a745e Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Roberto=20Bert=C3=B3?=
 <463349+robertoberto@users.noreply.github.com>
Date: Sat, 18 Oct 2025 04:48:02 +0000
Subject: [PATCH] fix(compat): ensure v1.ModelField gets proper BaseConfig

- add fallback to _get_v1().BaseConfig when model_config is None
- prevents AttributeError: 'NoneType' object has no attribute 'prepare_field'
---
 fastapi/utils.py | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fastapi/utils.py b/fastapi/utils.py
index 7cda73bae..5871a464b 100644
--- a/fastapi/utils.py
+++ b/fastapi/utils.py
@@ -96,13 +96,14 @@ def create_model_field(
     class_validators = class_validators or {}
 
     # Build v1 kwargs
+    v1_model_config = model_config or _get_v1().BaseConfig
     v1_kwargs: Dict[str, Any] = {
         "name": name,
         "type_": type_,
         "class_validators": class_validators,
         "default": default,
         "required": required,
-        "model_config": model_config,
+        "model_config": v1_model_config,
         "field_info": field_info,
         "alias": alias,
     }