Fixes golang/go#13456

Change-Id: I0b938f824c47b29ac2026eff83e61c2f227a6cc1
Reviewed-on: https://go-review.googlesource.com/47530
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>