Help with walker for nav

For some reason, this walker won’t work when I pass in 'echo ' => 0, it always prints the nav, and never returns the nav as a var. I can’t seem to figure out where it’s by passing that in wordpress. Any chance anyone can spot the problem?

function bones_main_nav() {
    $menu_args = array(
        'menu' => 'main_nav', /* menu name */
        'menu_class' => 'nav',
        'echo ' => 0,
        'theme_location' => 'main_nav', /* where in the theme it's assigned */
        'container' => 'false', /* container class */
        //'fallback_cb' => 'bones_main_nav_fallback', /* menu fallback */
        'depth' => '2', /* suppress lower levels for now */
        'walker' => new description_walker()
    );

    echo wp_nav_menu($menu_args);
    /*
    if(function_exists('new_beginnings')) {
        $topNav = wp_get_cache('topNav');
        if ($topNav === false) {
            $topNav = wp_nav_menu($menu_args);
            print_r($topNav);
            wp_set_cache('topNav', $topNav, 4000);
        }
        echo $topNav;
    } else {
        echo wp_nav_menu($menu_args);
    }
    */
}






// this is the fallback for header menu
function bones_main_nav_fallback() {
    // not calling this in case of multiple level pages - hope to add dropdown menu as an enhancement
    //wp_page_menu( 'show_home=Home&menu_class=menu' );
}

// this is the fallback for footer menu
function bones_footer_links_fallback() {
    /* you can put a default here if you like */
}

// Menu output mods
class description_walker extends Walker_Nav_Menu {
      function start_el(&$output, $item, $depth, $args) {
            global $wp_query;
            $indent = ( $depth ) ? str_repeat( "t", $depth ) : '';

            $class_names = $value = '';

            // If the item has children, add the dropdown class for bootstrap
            if ( $args->has_children ) {
                $class_names = "dropdown ";
            }

            $classes = empty( $item->classes ) ? array() : (array) $item->classes;

            $class_names .= join( ' ', apply_filters( 'nav_menu_css_class', array_filter( $classes ), $item ) );
            $class_names = ' class="'. esc_attr( $class_names ) . '"';

            $output .= $indent . '<li id="menu-item-'. $item->ID . '"' . $value . $class_names .'>';

            $attributes  = ! empty( $item->attr_title ) ? ' title="'  . esc_attr( $item->attr_title ) .'"' : '';
            $attributes .= ! empty( $item->target )     ? ' target="' . esc_attr( $item->target     ) .'"' : '';
            $attributes .= ! empty( $item->xfn )        ? ' rel="'    . esc_attr( $item->xfn        ) .'"' : '';
            $attributes .= ! empty( $item->url )        ? ' href="'   . esc_attr( $item->url        ) .'"' : '';
            // if the item has children add these two attributes to the anchor tag
            if ( $args->has_children ) {
                $attributes .= 'class="dropdown-toggle" data-toggle="dropdown"';
            }

            $item_output = $args->before;
            $item_output .= '<a'. $attributes .'>';
            $item_output .= $args->link_before .apply_filters( 'the_title', $item->title, $item->ID );
            $item_output .= $args->link_after;
            // if the item has children add the caret just before closing the anchor tag
            if ( $args->has_children ) {
                $item_output .= '<b class="caret"></b></a>';
            }
            else{
                $item_output .= '</a>';
            }
            $item_output .= $args->after;

            $output .= apply_filters( 'walker_nav_menu_start_el', $item_output, $item, $depth, $args );
            }

        function start_lvl(&$output, $depth) {
            $indent = str_repeat("t", $depth);
            $output .= "n$indent<ul class="dropdown-menu">n";
        }

        function display_element( $element, &$children_elements, $max_depth, $depth=0, $args, &$output )
            {
                $id_field = $this->db_fields['id'];
                if ( is_object( $args[0] ) ) {
                    $args[0]->has_children = ! empty( $children_elements[$element->$id_field] );
                }
                return parent::display_element( $element, $children_elements, $max_depth, $depth, $args, $output );
            }


}