Fixes #18535

Change-Id: I9e49d33ce357a534529a6b0fcdbc09ff4fa98622
Reviewed-on: https://go-review.googlesource.com/34920
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>