Fixes #16731.

Change-Id: I6d393357973d008ab7cf5fb264acb7d38c9354eb
Reviewed-on: https://go-review.googlesource.com/32104
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>