Fixes #16042.

Change-Id: I0a28aa004246b7b0ffaaab457e077ad9035363c2
Reviewed-on: https://go-review.googlesource.com/31932Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>