wp_dropdown_categories with slug in URL

Hellow guys !

I have made a wp_dropdown_categories to made a search/find by category but i can’t find out how to use ?cat=slug of my cat, in the url well i use the value of the selected to populate the url that’s good but what is output is not all the post for that category… but no search/find is done.
With something like ?cat=1 it work well but not with ?cat=slug name of my cat

<form id="category-select" class="category-select" action="<?php echo esc_url( home_url( '/episodes-au-hasard/' ) ); ?>" method="get">
		<?php
		$args = array(
			'show_option_all'  => 'TOUTES LES SéRIES',
			'show_count'       => false,
			'orderby'          => 'name',
			'echo'             => 0,
			'hide_if_empty'    => 1,
			'value_field'	   => 'slug',
			'hierarchical'     => 1,
			'depth'            => 1,
			'selected'         => htmlspecialchars($_GET["cat"]),
			'name'             => 'serie',
			'id'               => 'serie-cat-aleatoire',
		);
		?>
		<?php $select  = wp_dropdown_categories( $args ); ?>
		<?php $replace = "<select$1 onchange='return this.form.submit()'>"; ?>
		<?php $select  = preg_replace( '#<select([^>]*)>#', $replace, $select ); ?>
		<?php echo $select; ?>
	</form>

Thanks in advance !