Fixes #20924

Change-Id: If89f31da63cbea38d7e615a428b7b07629770a45
Reviewed-on: https://go-review.googlesource.com/47851
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tim Cooper <tim.cooper@layeh.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>