Fixes #16821.

Change-Id: I63d5f3d7cfba1c76259912d754025c5f3cbe4a56
Reviewed-on: https://go-review.googlesource.com/31573
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>