Add CSS Class to field

I am trying to add a new class to the textarea element for form number 7. Here is the code I am using:

// Assign a CSS class to a Gravity Form field
add_action("gform_field_css_class_7", "custom_class", 10, 3);
function custom_class($classes, $field, $form){
    if($field["type"] == "textarea"){
        $classes .= " ckeditor";
    }
    return $classes;
}

It isn't working....
What would I be doing wrong?