submenu error

goal: add sub menu below main menu:

1. added this to functions.php

function register_my_menus() {
  register_nav_menus(
    array(
      'Primary Menu' => __( 'matheson' ),
      'Expertise Menu' => __( 'expertise' )
    )
  );
}
add_action( 'init', 'register_my_menus' );

2. added this to header.php

<?php wp_nav_menu( array( 'theme_location' => 'expertise' ) ); ?>

3. get error:

Warning: in_array() expects parameter 2 to be array, string given in /home/michaelalexis/alexislawyers.com/wp-content/themes/matheson/functions.php on line 746

746 is: $element->classes[] = ( $element->current || in_array( 'current-menu-parent', $element->classes ) ) ? 'active' : '';

how do I fix the error and get submenu on site?

https://www.alexislawyers.com