Fixes #17584.

Change-Id: I3af31cc1f2e9c906f3b73e77f3b092624ba78fbe
Reviewed-on: https://go-review.googlesource.com/31939
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>