Fixes #4816.

Change-Id: Ibeaa69f57b7519d56df4ea357edf8d9dc2102ffe
Reviewed-on: https://go-review.googlesource.com/18851Reviewed-by: Rob Pike <r@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>