Fixes #17743.

Change-Id: Ib5afb6248bb060f2ad8dd3d5f78e95271af62a57
Reviewed-on: https://go-review.googlesource.com/33135
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Quentin Smith <quentin@golang.org>
Reviewed-by: Caleb Spare <cespare@gmail.com>