Fixes #16460.

Change-Id: Ie9d5f725d2d7e8210ab6f7604a5a05fc49f707de
Reviewed-on: https://go-review.googlesource.com/31331
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>