Fixes golang/go#17661

Change-Id: Iff004af969c694dde9bc3c56add3a419ab1beac7
Reviewed-on: https://go-review.googlesource.com/32373Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>