Found while tracking down #26855.

Change-Id: Ice137fe390820ba351e1c7439b6a9a1b3bdc966b
Reviewed-on: https://go-review.googlesource.com/136396
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>