Invalid Timestamp on edit with workbench and one revision

On a site with Scheduled Updates and Workbench Moderation and a scheduled update setup for the moderation state I get an uncaught exception when trying to edit a node that was published with only the default revision.

Reproduction Steps:

1. Install both modules
2. Setup a moderated content type can be published without going through "Needs revision"
3. Setup a Scheduled update field on the moderation state using latest revision runner and advanced settings (these may not matter but are the use case I've tried):
   • After update behavior: Archive updates
   • Invalid update behavior: Leave In Queue
   • Create New Revisions: Always create new revision
   • Run update as: Owner of revision
4. Create a new node.
5. During the creation process click the add new published draft button (or whatever you labeled the update) then click cancel.
6. Save the node to immediately publish.
7. Click the "create new draft" tab

The page shows:

The website encountered an unexpected error. Please try again later.

And the error log reads:

2016/08/17 15:33:42 [error] 923#923: *800 FastCGI sent in stderr: "PHP message: Uncaught PHP Exception Exception: "The timestamp must be numeric." at /var/www/vhosts/www.example.org/pub/core/lib/Drupal/Component/Datetime/DateTimePlus.php line 172" while reading response header from upstream, client: 192.168.2.1, server: www.example.org.sandbox$, request: "GET /node/51/edit HTTP/1.1", upstream: "fastcgi://unix:/var/run/php5-fpm.sock:", host: "www.example.org.sandbox", referrer: "http://www.example.org.sandbox/devel/node/51"

The exception is triggered by ScheduledUpdate.php in the label() method at line 260 (in current dev release). $this->get('update_timestamp')->getString() returns null, and the DateTime formatter will not accept. If you add a simple error trap to that function you get a blank entity attached:

  public function label() {
    /** @var \Drupal\Core\Datetime\DateFormatterInterface $formatter */
    $formatter = \Drupal::service('date.formatter');
    if($this->get('update_timestamp')->getString()){
      return $formatter->format($this->get('update_timestamp')->getString());
    } else {
      return False;
    }
  }

This may be a bug in Inline Entity Form, but since the invalid entity is a ScheduledUpdate it seems likely to a problem here. At the moment my solution just treats the symptom.