Fixes #16811.

Change-Id: I7d018015f691838482ccf845d621209b96935ba4
Reviewed-on: https://go-review.googlesource.com/31657
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Quentin Smith <quentin@golang.org>