diff --git a/windows/asm.s b/windows/asm.s
index d4ca868f..aaef9fe9 100644
--- a/windows/asm.s
+++ b/windows/asm.s
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build windows
+
 #include "textflag.h"
 
 TEXT ·use(SB),NOSPLIT,$0