diff --git a/animatediff/motion_module_ad.py b/animatediff/motion_module_ad.py
index 19fda16..1830266 100644
--- a/animatediff/motion_module_ad.py
+++ b/animatediff/motion_module_ad.py
@@ -1148,6 +1148,8 @@ def forward(
         view_options: Union[ContextOptions, None]=None,
         mm_kwargs: dict[str]=None,
     ):
+        if scale_masks is None:
+            scale_masks = [None] * len(self.attention_blocks)
         # make view_options None if context_length > video_length, or if equal and equal not allowed
         if view_options:
             if view_options.context_length > video_length: