Fixes #17030.

Change-Id: Ic7f237ac7553ae0176929056e64b01667ed59066
Reviewed-on: https://go-review.googlesource.com/31351
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>