Strict Standards

I’m debugging my theme. Everything works correctly when wp_debug is false but I get this error when I enable it. This file controls the dropdown menus in my theme.

Strict Standards: Declaration of Upbootwp_Walker_Nav_Menu::start_lvl() should be compatible with Walker_Nav_Menu::start_lvl(&$output, $depth = 0, $args = Array) in /homepages/1/d606045872/htdocs/clickandbuilds/RVAVideo/wp-content/themes/pgstrap/includes/nav_menu.php on line 152

Line 152 is the last line of code. Could anyone help me figure out what’s causing the problem?

<?php class Upbootwp_Walker_Nav_Menu extends Walker_Nav_Menu {
	public function __construct() {
		add_filter('nav_menu_css_class' , array($this, 'special_nav_class'), 10 , 2);
	}
	function special_nav_class($classes, $item){
		if(in_array('current-menu-item', $classes)){
			$classes[] = 'active ';
		}
		return $classes;
	}
    function start_lvl( &$output, $depth ) {
		$indent = str_repeat( "\t", $depth );
		$output	   .= "\n$indent<ul class=\"dropdown-menu bullet \">\n";
	}
	function start_el( &$output, $item, $depth = 0, $args = array(), $id = 0 ) {
		$indent = ( $depth ) ? str_repeat( "\t", $depth ) : '';
		$li_attributes = '';
		$class_names = $value = '';
		$classes = empty( $item->classes ) ? array() : (array) $item->classes;
		$classes[] = ($args->has_children) ? 'dropdown' : '';
		$classes[] = ($item->current || $item->current_item_ancestor) ? 'active' : '';
		$classes[] = 'menu-item-' . $item->ID;
		$class_names = join( ' ', apply_filters( 'nav_menu_css_class', array_filter( $classes ), $item, $args ) );
		$class_names = ' class="' . esc_attr( $class_names ) . '"';
		$id = apply_filters( 'nav_menu_item_id', 'menu-item-'. $item->ID, $item, $args );
		$id = strlen( $id ) ? ' id="' . esc_attr( $id ) . '"' : '';
		$output .= $indent . '<li' . $id . $value . $class_names . $li_attributes . '>';
		$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        ) .'"' : '';
		$attributes .= ($args->has_children) 	    ? ' 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 ) . $args->link_after;
		$item_output .= ($args->has_children) ? ' <b class="caret"></b></a>' : '</a>';
		$item_output .= $args->after;
		$output .= apply_filters( 'walker_nav_menu_start_el', $item_output, $item, $depth, $args );
	}
	function display_element( $element, &$children_elements, $max_depth, $depth=0, $args, &$output ) {
		if ( !$element )
			return;
		$id_field = $this->db_fields['id'];
		if ( is_array( $args[0] ) )
			$args[0]['has_children'] = ! empty( $children_elements[$element->$id_field] );
		else if ( is_object( $args[0] ) )
			$args[0]->has_children = ! empty( $children_elements[$element->$id_field] );
		$cb_args = array_merge( array(&$output, $element, $depth), $args);
		call_user_func_array(array(&$this, 'start_el'), $cb_args);
		$id = $element->$id_field;
		if ( ($max_depth == 0 || $max_depth > $depth+1 ) && isset( $children_elements[$id]) ) {
			foreach( $children_elements[ $id ] as $child ){
				if ( !isset($newlevel) ) {
					$newlevel = true;
					$cb_args = array_merge( array(&$output, $depth), $args);
					call_user_func_array(array(&$this, 'start_lvl'), $cb_args);
				}
				$this->display_element( $child, $children_elements, $max_depth, $depth + 1, $args, $output );
			}
				unset( $children_elements[ $id ] );
		}
		if ( isset($newlevel) && $newlevel ){
			$cb_args = array_merge( array(&$output, $depth), $args);
			call_user_func_array(array(&$this, 'end_lvl'), $cb_args);
		}
		$cb_args = array_merge( array(&$output, $element, $depth), $args);
		call_user_func_array(array(&$this, 'end_el'), $cb_args);
	}
}