diff --git a/webp/decode.go b/webp/decode.go
index 134307e..488e0ab 100644
--- a/webp/decode.go
+++ b/webp/decode.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 webp implements a decoder for WEBP images.
 //
 // WEBP is defined at:
diff --git a/webp/decode_test.go b/webp/decode_test.go
index ad65b10..b27468a 100644
--- a/webp/decode_test.go
+++ b/webp/decode_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 webp
 
 import (