Fixes golang/go#22301.

Change-Id: I29f4f8806e10aad59cf7f984fd346acc216b0fd0
Reviewed-on: https://go-review.googlesource.com/79855
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>