Fixes #17689.

Change-Id: I45a14e6bf4b2647431105f3e0b63b7076b6655d2
Reviewed-on: https://go-review.googlesource.com/32635
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>