Fixes #20266.

Change-Id: I51383820880e3d3566ef3d70650a0863756003ba
Reviewed-on: https://go-review.googlesource.com/44291
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>