Filter custom post types in archive


I want to filter my custom posts (pump) with a custom filter form on it’s archive page (archive-pump.php).

So, I wrote the form markup:

<form method="GET">     <label>Series</label>     <?php         if( $  terms = get_terms( array( 'taxonomy' => 'series', 'orderby' => 'name' ) ) ) :             echo '<select name="series">';             foreach ( $  terms as $  term ) :                 echo '<option value="' . $  term->term_id . '">' . $  term->name . '</option>'; // ID of the category as the value of an option             endforeach;             echo '</select>';         endif;     ?>     <button type="submit">Apply filter</button> </form> 

And I have this to output my custom posts:

<?php if(have_posts()) : while(have_posts()) : the_post(); ?>     <?php the_title( '', '', true ); ?> <?php endwhile; endif; ?> 

When I open my page (localhost/project/pumps) it looks fine. But when I submit my form I’m getting an 404 page.

I maybe need an seperate query to fetch all the $ _GET data. But I’m not getting to the step because of the 404 error.

What am I doing wrong? Thank you!