Fixes #15673

Change-Id: Ib36d8db3299a93d92665dbde012d52c2c5332ac0
Reviewed-on: https://go-review.googlesource.com/23253Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>