Fixes golang/go#23639

Change-Id: I7793dc50de35791a7c8af1966bb2ee4a36a2ac1c
Reviewed-on: https://go-review.googlesource.com/91375
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>