diff --git a/python/sglang/bench_one_batch.py b/python/sglang/bench_one_batch.py
index a4bed02ac..ad154fd50 100644
--- a/python/sglang/bench_one_batch.py
+++ b/python/sglang/bench_one_batch.py
@@ -393,7 +393,7 @@ def prepare_extend_inputs_for_correctness_test(
 ):
     for i in range(len(reqs)):
         req: Req = reqs[i]
-        req.full_untruncated_fill_ids += input_ids[i][bench_args.cut_len :]
+        req.full_untruncated_fill_ids.extend(input_ids[i][bench_args.cut_len :])
         req.fill_len = len(req.full_untruncated_fill_ids)
         if model_runner is not None:
             # Use req.req_pool_idx instead of i to handle slot 0 padding correctly