Fixes #16959.

Change-Id: Ibbb28fdf26c53788a0edb3e3ea54ec030fa2a8cf
Reviewed-on: https://go-review.googlesource.com/31611
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>