Fixes #18993

Change-Id: Ia1fa20b6d82384b07e9ba5512b909439e0bec2a5
Reviewed-on: https://go-review.googlesource.com/36611
Run-TryBot: Caleb Spare <cespare@gmail.com>
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>