Made my nav menu customizable, now shows bullets

The theme I’m using didn’t have a customizable menu option, so I had to make it as such. Everything looks okay except that the menu now shows unordered list bullets.

Here is the code from my functions.php file:

// Custom menus

function register_my_menus() {
  register_nav_menus(
    array( 'header-menu' => __( 'Header Menu' ), 'extra-menu' => __( 'Extra Menu' ))
  );
}

add_action( 'init', 'register_my_menus' );

It’s located right at the very end of the code.

Here is the code from my header.php file

<ul class="topnav">

<?php wp_nav_menu( array( 'theme_location' => 'header-menu' ) ); ?> 

</ul>

Can someone help me remove the bullets? ??