Fixes #16993.

Change-Id: Ibe406f97d2a49acae94531d969c56dbac8ce53b2
Reviewed-on: https://go-review.googlesource.com/28511
Run-TryBot: Minux Ma <minux@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>