Error with Walker_Nav_Menu

Hello,

I have received this error:

Strict standards: Declaration of DropDown_Nav_Menu::start_lvl() should be compatible with Walker_Nav_Menu::start_lvl(&$output, $depth = 0, $args = Array) in C:\wamp\www\home\wordpress\wp-content\themes\tourpackage-v2-00\include\plugin\dropdown-menus.php on line 173

Please find below code from the dropdown-menus.php

function start_el( &$output, $item, $depth, $args ) {
		global $wp_query;
		$indent = ( $depth ) ? str_repeat( "\t", $depth ) : '';

		$class_names = $value = '';

		$classes = empty( $item->classes ) ? array() : (array) $item->classes;
		$classes[] = 'menu-item-' . $item->ID;
		$classes[] = 'menu-item-depth-' . $depth;

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

		// select current item
		$selected = in_array( 'current-menu-item', $classes ) ? ' selected="selected"' : '';

		$output .= $indent . '<option' . $class_names .' value="'. $item->url .'"'. $selected .'>';

		// push sub-menu items in as we can't nest optgroups
		$indent_string = str_repeat( apply_filters( 'dropdown_menus_indent_string', $args->indent_string, $item, $depth, $args ), ( $depth ) ? $depth : 0 );
		$indent_string .= !empty( $indent_string ) ? apply_filters( 'dropdown_menus_indent_after', $args->indent_after, $item, $depth, $args ) : '';

		$item_output = $args->before . $indent_string;
		$item_output .= $args->link_before . apply_filters( 'the_title', $item->title, $item->ID ) . $args->link_after;
		$item_output .= $args->after;

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

	/**
	 * @see Walker::end_el()
	 * @since 3.0.0
	 *
	 * @param string $output Passed by reference. Used to append additional content.
	 * @param object $item Page data object. Not used.
	 * @param int $depth Depth of page. Not Used.
	 */
	function end_el( &$output, $item, $depth ) {
		$output .= apply_filters( 'walker_nav_menu_dropdown_end_el', "</option>\n", $item, $depth);
	}
}

Please advise