diff --git a/src/fairseq2/generation/step_processor.py b/src/fairseq2/generation/step_processor.py
index c807b47ef..2e0e3b2df 100644
--- a/src/fairseq2/generation/step_processor.py
+++ b/src/fairseq2/generation/step_processor.py
@@ -106,7 +106,7 @@ def __call__(self, seqs: Tensor, probs: Tensor, lprob: bool = False) -> None:
 
         if (len_delta := banned_prefix_len - seqs.size(1)) > 0:
             # (N, S) -> (N, S_pre)
-            seqs = pad(seqs, (len_delta, -1))
+            seqs = pad(seqs, (len_delta, 0), value=-1)
         elif len_delta < 0:
             # (N, S) -> (N, S_pre)
             seqs = seqs[:, -banned_prefix_len:]