Trying to list terms of a custom taxonomy using get_categories

get_categories()

fetches taxonomies of type ‘categories’ particularly http://core.trac.wordpress.org/browser/tags/3.6/wp-includes/category.php#L0,
to fetch custom taxonomy you should use get_terms() instead, here http://codex.wordpress.org/Function_Reference/get_terms

$terms = get_terms( 'partner-cat', 'orderby=count&hide_empty=0' );
$count = count($terms);
if ( $count > 0 ){
 echo "<ul>";
 foreach ( $terms as $term ) {
   echo "<li>" . $term->name . "</li>";

 }
 echo "</ul>";
}

Make sure you specify the proper slug you registered for taxonomy and change WP_DEBUG to true in your config file to check for further errors, as you might be fetching the taxonomy before you are registering it and hence no results in that case you’d get an error.

can you paste your code for registering taxonomy?