diff --git a/egs/mls_english/ASR/zipformer/streaming_decode.py b/egs/mls_english/ASR/zipformer/streaming_decode.py
index 7e3199e09..e8e330481 100755
--- a/egs/mls_english/ASR/zipformer/streaming_decode.py
+++ b/egs/mls_english/ASR/zipformer/streaming_decode.py
@@ -386,12 +386,12 @@ def streaming_forward(
     src_key_padding_mask = make_pad_mask(x_lens)
 
     # processed_mask is used to mask out initial states
-    processed_mask = torch.arange(left_context_len, device=x.device).expand(
+    processed_lens = states[-1]  # (batch,)
+    idx = torch.arange(left_context_len, device=x.device).unsqueeze(0).expand(
         x.size(0), left_context_len
     )
-    processed_lens = states[-1]  # (batch,)
-    # (batch, left_context_size)
-    processed_mask = (processed_lens.unsqueeze(1) <= processed_mask).flip(1)
+    # True means padding positions (not yet available in cache).
+    processed_mask = idx >= processed_lens.unsqueeze(1)
     # Update processed lengths
     new_processed_lens = processed_lens + x_lens