diff --git a/fastapi/dependencies/utils.py b/fastapi/dependencies/utils.py
index 0e3342ffc..e735b17cd 100644
--- a/fastapi/dependencies/utils.py
+++ b/fastapi/dependencies/utils.py
@@ -196,7 +196,7 @@ def _get_flat_fields_from_params(fields: List[ModelField]) -> List[ModelField]:
 
       fields_to_extract = []
     for f in fields:
-        if _is_model_class(first_field.type_):
+        if _is_model_class(f.type_):
             fields_to_extract.extend(get_cached_model_fields(f.type_))
         else:
             fields_to_extract.append(f)