diff --git a/util/misc.py b/util/misc.py
index 1d4e5eb13..78f2182a6 100644
--- a/util/misc.py
+++ b/util/misc.py
@@ -290,7 +290,6 @@ def to(self, device):
         cast_tensor = self.tensors.to(device)
         mask = self.mask
         if mask is not None:
-            assert mask is not None
             cast_mask = mask.to(device)
         else:
             cast_mask = None