diff --git a/python/sglang/kernels/ops/speculative/topk1.py b/python/sglang/kernels/ops/speculative/topk1.py
index a63a25617..93f658f2a 100644
--- a/python/sglang/kernels/ops/speculative/topk1.py
+++ b/python/sglang/kernels/ops/speculative/topk1.py
@@ -27,6 +27,8 @@ def _draft_topk1_partial_argmax_kernel(
         mask=mask,
         other=-float("inf"),
     ).to(tl.float32)
+    # Keep NaNs on valid lanes from selecting the masked tail.
+    vals = tl.where(vals == vals, vals, -1e30)
 
     max_val = tl.max(vals, axis=0)
     local_index = tl.argmax(vals, axis=0)