diff --git a/fastapi/dependencies/utils.py b/fastapi/dependencies/utils.py
index dad7e6562..97c1f7eaf 100644
--- a/fastapi/dependencies/utils.py
+++ b/fastapi/dependencies/utils.py
@@ -254,6 +254,8 @@ def is_scalar_mapping_field(field: ModelField) -> bool:
     if (field.shape in mapping_shapes) and not lenient_issubclass(
         field.type_, BaseModel
     ):
+        if field.sub_fields is None:
+            return False
         for sub_field in field.sub_fields:
             if not is_scalar_field(sub_field):
                 return False
@@ -265,6 +267,8 @@ def is_scalar_sequence_mapping_field(field: ModelField) -> bool:
     if (field.shape in mapping_shapes) and not lenient_issubclass(
         field.type_, BaseModel
     ):
+        if field.sub_fields is None:
+            return False
         for sub_field in field.sub_fields:
             if not is_scalar_sequence_field(sub_field):
                 return False