Fixes #21522.

Change-Id: Ifec1681b265576c47a4d736f6f124cc25485c593
Reviewed-on: https://go-review.googlesource.com/57011
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>