diff --git a/slime/backends/megatron_utils/model.py b/slime/backends/megatron_utils/model.py
index ffe3f06ba2..3b70162446 100644
--- a/slime/backends/megatron_utils/model.py
+++ b/slime/backends/megatron_utils/model.py
@@ -226,6 +226,7 @@ def forward_step(
         response_lengths = batch["response_lengths"]
         forward_kwargs = {
             "input_ids": tokens,
+            "position_ids": None,
             "attention_mask": None,
             "labels": None,
             "packed_seq_params": packed_seq_params,