Fixes #13347.

Change-Id: I591a80a1566ce70efb5f68e3ad69e7e3ab98cd9b
Reviewed-on: https://go-review.googlesource.com/23102Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>