Fixes #14201.

Change-Id: Ib61f8c00cae72463f59b90ae199fbdc1e7422a79
Reviewed-on: https://go-review.googlesource.com/19174
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Hudson-Doyle <michael.hudson@canonical.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>