Fixes #21587.

Change-Id: I47eb181d65da67a3b530c7f8acac9c0c619ea474
Reviewed-on: https://go-review.googlesource.com/83796
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>