diff --git a/windows/env_unset.go b/windows/env_unset.go
index 999ffac4..4ed03aee 100644
--- a/windows/env_unset.go
+++ b/windows/env_unset.go
@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build windows
 // +build go1.4
 
 package windows