diff --git a/go/gcimporter15/bexport.go b/go/gcimporter15/bexport.go
index 53b93106..8eb048eb 100644
--- a/go/gcimporter15/bexport.go
+++ b/go/gcimporter15/bexport.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 go1.6
+
 // Binary package export.
 // This file was derived from $GOROOT/src/cmd/compile/internal/gc/bexport.go;
 // see that file for specification of the format.
diff --git a/go/gcimporter15/bexport_test.go b/go/gcimporter15/bexport_test.go
index 956ed3aa..fea4536f 100644
--- a/go/gcimporter15/bexport_test.go
+++ b/go/gcimporter15/bexport_test.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 go1.6
+
 package gcimporter_test
 
 import (