Fixes issue golang/go#17563

Change-Id: Ibb41ea9419907193526cc601f6afd07d8689b1fe
Reviewed-on: https://go-review.googlesource.com/31810Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>