Fixes #18301.

Change-Id: I990c105904ab62f2225d671bbc10209ec51b12e2
Reviewed-on: https://go-review.googlesource.com/34371
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>