Define variables for specific template
You should put your template in your module as well.

my_theme/templates/my-entities-property.tpl.php


Make sure to update your theme function like this :

function my_entities_theme($existing, $type, $theme, $path) {
  return array(
    'my_entities_property' =&gt; array(
      'variables' =&gt; array('element' =&gt; null),
      'template' =&gt; 'my-entities-property',
      'path' =&gt; drupal_get_path('module', 'my_entities') . '/templates'
    )
  );
}


Try to use THEME_preprocess_my_entities_property(&amp;$variables) in template.php instead.