Fixes #20365.

Change-Id: If1a4866193cff3bc836d8bbf18b6a1f5deb9808d
Reviewed-on: https://go-review.googlesource.com/43550
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Alessandro Arzilli <alessandro.arzilli@gmail.com>