Fixes #13757.

Change-Id: I1b52593df8df0e98ce7342767eb34eccecc11761
Reviewed-on: https://go-review.googlesource.com/18854
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>