Fixes #14519.

Change-Id: I8f78f67a463e6467e09df90446f7ebd28789d6c9
Reviewed-on: https://go-review.googlesource.com/19933Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-on: https://go-review.googlesource.com/22071
Run-TryBot: Andrew Gerrand <adg@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>