Fixes #22258.

Change-Id: I43e68f1cf3163e1a041ebff2734ff2cb7943f695
Reviewed-on: https://go-review.googlesource.com/71431Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>