Fixes #13647.

Change-Id: I28df7ade9b5abd79ce6b9c3d14ceaa988e86fc01
Reviewed-on: https://go-review.googlesource.com/18642
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>