Lock in fix for #22738, submitted in CL 78031.

Fixes #22738.

Change-Id: I6896feb158569e3f12fa7055387cbd7caad29ef4
Reviewed-on: https://go-review.googlesource.com/80635
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>