diff --git a/fastapi/utils.py b/fastapi/utils.py
index b9301499a..9d720feb3 100644
--- a/fastapi/utils.py
+++ b/fastapi/utils.py
@@ -147,15 +147,15 @@ def generate_unique_id(route: "APIRoute") -> str:
 
 
 def deep_dict_update(main_dict: Dict[Any, Any], update_dict: Dict[Any, Any]) -> None:
-    for key in update_dict:
+    for key, value in update_dict.items():
         if (
             key in main_dict
             and isinstance(main_dict[key], dict)
-            and isinstance(update_dict[key], dict)
+            and isinstance(value, dict)
         ):
-            deep_dict_update(main_dict[key], update_dict[key])
+            deep_dict_update(main_dict[key], value)
         else:
-            main_dict[key] = update_dict[key]
+            main_dict[key] = value
 
 
 def get_value_or_default(