diff --git a/python/sglang/bench_one_batch.py b/python/sglang/bench_one_batch.py
index 1022ea3d6..e3a2be61e 100644
--- a/python/sglang/bench_one_batch.py
+++ b/python/sglang/bench_one_batch.py
@@ -383,8 +383,9 @@ def prepare_extend_inputs_for_correctness_test(
 ):
     for i in range(len(reqs)):
         req: Req = reqs[i]
-        req.fill_ids += input_ids[i][bench_args.cut_len :]
+        req.fill_ids.extend(input_ids[i][bench_args.cut_len :])
         if model_runner is not None:
+            # Use req.req_pool_idx instead of i to handle slot 0 padding correctly
             req.prefix_indices = model_runner.req_to_token_pool.req_to_token[
                 req.req_pool_idx, : bench_args.cut_len
             ].to(req.prefix_indices.dtype)