Fixes #13977.

Change-Id: Icf54b4d2d746d30da207d1e17c975d18188b1cf8
Reviewed-on: https://go-review.googlesource.com/18702
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>