diff --git a/root_stub.go b/root_stub.go
index 1903eed..18dcb02 100644
--- a/root_stub.go
+++ b/root_stub.go
@@ -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 plan9
+
 package tls
 
 func initDefaultRoots() {
diff --git a/root_unix.go b/root_unix.go
index 1b9aeb0..5bbd982 100644
--- a/root_unix.go
+++ b/root_unix.go
@@ -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 freebsd linux openbsd netbsd
+
 package tls
 
 import (