Fixes golang/go#19262

Change-Id: I67ceb187cd68e90a9387a706a89aafded0befe53
Reviewed-on: https://go-review.googlesource.com/37344Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Matt Layher <mdlayher@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>