Undefined variable: entities_count in InlineEntityFormMultiple

Testing IEF on Drupal 8 beta 12 I'm experiencing behavior where saving the IEF does not complete, returning with the open form and no displayed error. I had tested IEF on beta 11 and it had been successfully submitting. I'm running the latest dev release at the time of this posting. Console shows an AJAX 500 error:

Failed to load resource: the server responded with a status of 500 (Internal Server Error)

Uncaught AjaxError: 
An AJAX HTTP error occurred.
HTTP Result Code: 500
Debugging information follows.
Path: /node/add/issue?ajax_form=1&_wrapper_format=drupal_ajax
StatusText: Internal Server Error
ResponseText: 

If I try to save the parent form with IEF open and filled out, the page will WSOD. Watch dog has this notice logged:
<em class="placeholder">Notice</em>: Undefined variable: entities_count in <em class="placeholder">Drupal\inline_entity_form\Plugin\Field\FieldWidget\InlineEntityFormMultiple-&gt;formElement()</em> (line <em class="placeholder">298</em> of <em class="placeholder">modules/inline_entity_form/src/Plugin/Field/FieldWidget/InlineEntityFormMultiple.php</em>)