diff --git a/class-wp-bootstrap-navwalker.php b/class-wp-bootstrap-navwalker.php
index 6fdd209..752d3c9 100644
--- a/class-wp-bootstrap-navwalker.php
+++ b/class-wp-bootstrap-navwalker.php
@@ -19,39 +19,39 @@ class WP_Bootstrap_Navwalker extends Walker_Nav_Menu
     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->walker->has_children) ? 'dropdown' : '';
+        $classes[] = (isset($args->walker) && $args->walker->has_children) ? 'dropdown' : '';
         $classes[] = 'nav-item';
         $classes[] = 'nav-item-' . $item->ID;
-        if ($depth && $args->walker->has_children) {
+        if ($depth && isset($args->walker) && $args->walker->has_children) {
             $classes[] = 'dropdown-submenu';
         }
-
+    
         $class_names = join(' ', apply_filters('nav_menu_css_class', array_filter($classes), $item, $args));
         $class_names = ' class="' . esc_attr($class_names) . '"';
-
+    
         $id = apply_filters('nav_menu_item_id', 'nav-menu-item-' . $item->ID, $item, $args);
         $id = $id ? ' id="' . esc_attr($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->walker->has_children) ? ' class="nav-link dropdown-toggle" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false"' : ' class="nav-link"';
-
-        $item_output = $args->before;
+    
+        $attributes .= (isset($args->walker) && $args->walker->has_children) ? ' class="nav-link dropdown-toggle" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false"' : ' class="nav-link"';
+    
+        $item_output = isset($args->before) ? $args->before : '';
         $item_output .= '<a' . $attributes . '>';
-        $item_output .= $args->link_before . apply_filters('the_title', $item->title, $item->ID) . $args->link_after;
+        $item_output .= (isset($args->link_before) ? $args->link_before : '') . apply_filters('the_title', $item->title, $item->ID) . (isset($args->link_after) ? $args->link_after : '');
         $item_output .= '</a>';
-        $item_output .= $args->after;
-
+        $item_output .= isset($args->after) ? $args->after : '';
+    
         $output .= apply_filters('walker_nav_menu_start_el', $item_output, $item, $depth, $args);
     }
 }