Was part of #13822 but not in the first message, so I missed it.

Fixes #13822 again.

Change-Id: I775004fa8d47b6af293124605521ec396573e267
Reviewed-on: https://go-review.googlesource.com/18900
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>