No integer key for result

[wohnlice]

This is more of a question than an issue. A user deleted some page which is still being found in elasticsearch and resulting in the following, from brain.py

Got not integer key for result: {u'_score': 4.167767, u'_type': u'portal_catalog', u'_id': u'c3d6e66a2b924125b367a27c475b65de', u'fields': {u'path.path': [u'/db11/webdev/projects/cpfp-drupal']}, u'_index': u'plone-db11-webdev-portal_catalog_1'}

The error is swallowed by an exception clause and returns a value of None, which I then ignore in my search results. But I don't understand, what would cause this desync? Shouldn't it be removed from elasticsearch when the page is deleted in Plone? I am unable to reproduce this when deleting other pages, so I don't know what could cause this case to happen.