Fixes #15104.

Change-Id: I9ddfbbf39ef0a873b703ee3e04fbb7d1192f5f39
Reviewed-on: https://go-review.googlesource.com/21581
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>