TypeError: $(...).autocomplete is not a function

I am getting the error TypeError: $(...).autocomplete is not a function when using the following code inside a Drupal module.

jQuery(document).ready(function($) {
        $("#search_text").autocomplete({
            source:results,
            minLength:2,
            position: { offset:'-30 0' },  
            select: function(event, ui ) { 
                    goTo(ui.item.value);
                    return false;
            }        
    }); 
});

jQuery is definitely loaded, and I have tried using a different variable for $ - any ideas what else might be the problem?

(Edit) Drupal specific answer for autocomplete:

drupal_add_library('system', 'ui.autocomplete');