Variable "url" does not exist.

[snoop168]

When using the source option to call a javascript function I get the above error. I declare the field like so

        $builder->add('street', 'entity_typeahead', array(
            'class'  => 'AppBundle:Street',
            'render' => 'name',
            'source' => 'test',
            'minLength' => 1,
            'callback' => 'test2'

        ))

If I simply include the 'route' option it seems to resolve the issue and still calls the JS function but not sure why it would be required. Documentation says just one or the other.