Fixes #17720

Change-Id: Ib95c230deef3934db729856c17908f8e5a1e2b7f
Reviewed-on: https://go-review.googlesource.com/33145
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Rob Pike <r@golang.org>