Fixes #14124.

Change-Id: I9a694c402e613d27701e7e41640af357c373edea
Reviewed-on: https://go-review.googlesource.com/18959Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>