Fixes #12540.

Change-Id: I7893fdc023145b0aca4b4c7df7e08e47edcf5bba
Reviewed-on: https://go-review.googlesource.com/16902
Run-TryBot: Minux Ma <minux@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>