Fixes #21677.

Change-Id: I869dee5f43df5d87d86922681726297e3024c562
Reviewed-on: https://go-review.googlesource.com/59810
Run-TryBot: Michael Munday <mike.munday@ibm.com>
Reviewed-by: Martin Möhrmann <moehrmann@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>