Fixes golang/go#18632.

Change-Id: I9d07937dc93f860b09a99a86fe517b85f4dcd1b2
Reviewed-on: https://go-review.googlesource.com/89495Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>