wp_nav_menu没有显示在单个帖子页面上

I am using Twentytwelve theme and everything is fine but navigation menu is not displaying in single post page whereas the navigation menus are displaying in every other page. This is the snippet I am using in header.php to display menu.

    wp_nav_menu( array(  
       'theme_location'  => 'primary', 
       'menu_class'      => 'nav-menu',
       'container_class' => '',
       'container'       => false,
       'fallback_cb'     => false  ) 
    );