Fixes #17148.

Change-Id: I4c66aa0733c249ee6019d1c4e802a7e30457d4b6
Reviewed-on: https://go-review.googlesource.com/32030
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>