Fixes #20541.

Change-Id: Ifdfdf3616482b71761daf6d114b779a8ec532051
Reviewed-on: https://go-review.googlesource.com/44495
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>