diff --git a/src/pkg/runtime/netpoll.goc b/src/pkg/runtime/netpoll.goc
index 2c61952700..84810003ea 100644
--- a/src/pkg/runtime/netpoll.goc
+++ b/src/pkg/runtime/netpoll.goc
@@ -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 darwin
+
 package net
 
 #include "runtime.h"