WP query 'category_name' not working when 1 category selected

Hi guys,
I have a javascript script that write a url including queries for filters, so when pressed, those queries are applied. This is used in combination with the following php code;

<?php  

	$paged = (get_query_var('paged')) ? get_query_var('paged') : 1;

	if( !empty($_GET['orderby']) ) $orderby=$_GET['orderby'];
	else $orderby = 'Date'; // default
	if( !empty($_GET['tag']) ) $tags=$_GET['tag'];
	else $tags = ''; // default
	if( !empty($_GET['colours']) ) $colours=$_GET['colours'];
	else $colours = ''; // default
	if( !empty($_GET['category_name']) ) $catname=$_GET['category_name'];
	else $catname = ''; // default
	if( !empty($_GET['author']) ) $author=$_GET['author'];
	else $author = ''; // default	

$args = array(
      'meta_key' => '_post_like_count',
      'orderby' => $orderby,
      'category_name' => $catname,
      'tag' => $tags,
      'colours' => $colours,
      'author' => $author,
      'paged' => $paged
    );
?>

This all works amazingly, yet when I only activate 1 category,
i.e the url is index.php/?orderby=date&order=DESC&category_name=Graphics the query grabs all posts from the blog.
This does not happen when I activate 2 categories, it grabs both categories posts.
Does anyone know why this is happening?