Fixes #15610.

Change-Id: Idbc8a9b328b92034d53b8009471678a166d5cf3f
Reviewed-on: https://go-review.googlesource.com/31588
TryBot-Result: Gobot Gobot <gobot@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Quentin Smith <quentin@golang.org>