Fixes #19753.

Change-Id: Ib20a69b1d0bcc42aa9e924918bcb578d6a560a31
Reviewed-on: https://go-review.googlesource.com/38742
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>