How to create entity reference field from custom entity to another custom entity? I'm on Drupal 8 and I created a custom entity custom_entity_one following the Drupal 8 examples, the content_entity_example module. 
Now, in another custom entity, custom_entity_two I'm trying to create a BaseFieldDefinition like so:

$fields['cid'] = BaseFieldDefinition::create('entity_reference')
  -&gt;setLabel(t('custom_entity_one ID'))
  -&gt;setSettings(array(
    'target_type' =&gt; 'custom_entity_one',
    'multiple' =&gt; TRUE,
    'handler' =&gt; 'default'
  ))
  -&gt;setDisplayOptions('view', array(
    'label' =&gt; 'above',
    'type' =&gt; 'id',
    'weight' =&gt; -1,
  ))
  -&gt;setDisplayOptions('form', array(
    'type' =&gt; 'entity_reference_autocomplete',
    'settings' =&gt; array(
      'match_operator' =&gt; 'CONTAINS',
      'size' =&gt; 60,
      'placeholder' =&gt; '',
    ),
    'weight' =&gt; -1,
  ))
  -&gt;setDisplayConfigurable('form', TRUE)
  -&gt;setDisplayConfigurable('view', TRUE);


In the add form of the custom_entity_two, the reference field appears, however, it doesn't find any value, even though I have multiple entities of type custom_entity_one. 

I thought that it will look for the label propriety of the entity, but it doesn't seem so. What I am missing ? Should I create a custom handler ? I'm not very sure how.
Yes, I was right, right after I posted the question I checked if I'm missing the entity_key label, and I was. I added it in the entity annotation definition, like so:

entity_keys = {
  "id" = "id",
  "uuid" = "uuid",
  "label" = "title",
},