Fixes #15271

Change-Id: I28e3fb5bde1e6fd5b263b1434873b8ce051aee97
Reviewed-on: https://go-review.googlesource.com/22083
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>