Fixes #15920.

Change-Id: I78cd79b91a58d0f7218b80f9445417f4ee071a6e
Reviewed-on: https://go-review.googlesource.com/23606Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>