Fixes #15310.

Change-Id: I588b3c630a20a6878f7cd00f9af29b1dd8a4abf6
Reviewed-on: https://go-review.googlesource.com/22100
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>