Fixes #11681.

Change-Id: I679d71ed25ac585af7d43611be01c1a0c4807871
Reviewed-on: https://go-review.googlesource.com/12554
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>