Fixes #17038.

Change-Id: Iaf6294361050040830af1d60cd48f263223d9356
Reviewed-on: https://go-review.googlesource.com/28966
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>