Fixes #15463.

Change-Id: Ic85886861c650ffcb71240d847941534152b92bc
Reviewed-on: https://go-review.googlesource.com/22540
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>