fixes #21635

Change-Id: I78716d47b6a930074e5394f7171d5f545dfba0bc
Reviewed-on: https://go-review.googlesource.com/59690Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Rob Pike <r@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>