Fixes #18863.

Change-Id: I0723563cd23728b0d43ebcc25979bf8d21e2a72c
Reviewed-on: https://go-review.googlesource.com/36427
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-on: https://go-review.googlesource.com/36536Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>