Fixes #8206.

Change-Id: Iec1026ecc586495f5c9562cc84b3240c71d53da5
Reviewed-on: https://go-review.googlesource.com/22164
Run-TryBot: David du Colombier <0intro@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>