Fixes #15122

Change-Id: Ie2c802d78aea731e25bf4b193b3c2e4c884e0573
Reviewed-on: https://go-review.googlesource.com/21524
Run-TryBot: Dave Cheney <dave@cheney.net>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>