Fixes #15121

Change-Id: I651521743c56244c55eda5762905889d7e06887a
Reviewed-on: https://go-review.googlesource.com/21521Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>