Fixes #17600.

Change-Id: I7aa0eb0dd959da031b6039b51f07db668d4fb468
Reviewed-on: https://go-review.googlesource.com/33131
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Gudger <igudger@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>