If the optional fields are not set, trying to cast to the Object subtype fails.

Change-Id: I386c6b23cbd3d282a72093dfc78966d0046578c3
Reviewed-on: https://chromium-review.googlesource.com/1073307Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53358}