Fixes #15661.

Change-Id: Ic3a8296fc7107f491880900ef52563e52caca1a3
Reviewed-on: https://go-review.googlesource.com/23615
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>