diff --git a/bench/attn/flash_attn_bench.py b/bench/attn/flash_attn_bench.py
index d25af5af..71288e7b 100644
--- a/bench/attn/flash_attn_bench.py
+++ b/bench/attn/flash_attn_bench.py
@@ -126,7 +126,7 @@ def benchmark(
     ws = (window_size, 0) if window_size > 0 else None
 
     def fn():
-        flash_attn_func(Q, K, V, causal=causal, window_size=ws)
+        flash_attn_func(Q, K, V, causal=causal, window_size=ws if ws is not None else (-1, -1))
 
     ms = do_bench(fn, (), opts)