Fixes #13559.

Change-Id: I6fe8b5083192e8eb6c1b3ca1919fde81a00ccb7e
Reviewed-on: https://go-review.googlesource.com/17695
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>