Fixes #16243.

Change-Id: I207d1e8aa48abe453a23c709ccf4f8e07368595b
Reviewed-on: https://go-review.googlesource.com/24648
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>