WordPress giving a 404 page when passing a year argument different than the curr

On this page you will see I’m listing events that fall in certain months: https://conciergemaps.ch/events/

This is on WordPress, When you select a certain month and year it appends the URL with two arguments (month and year i.e. ?month=October&year=2014). The problem is if the year is anything other than the current year (2014), it gives a 404 error. I use the below code to only show the correct events that fall under the selected month (they’re a custom post type):

$events_month = sanitize_text_field($_GET["month"]);
    $events_year = sanitize_text_field($_GET["year"]);

    if (empty($events_month) || empty($events_year)) {

    	$events_month = date('F');
    	$events_year = date('Y');

    }

    $ts = strtotime("$events_month $events_year");

    $month_start_date = date('Ym01', $ts);
    $month_end_date = date('Ymt', $ts);

    $args = array(
    	'post_type' => 'events',
    	'posts_per_page' => 50,
    	'order' => 'ASC',
    	'orderby' => 'meta_value_num',
    	'meta_key' => 'start_date',
    	'meta_query' => array(
    	    array(
    	        'key' => 'start_date',
    	        'value' => $month_end_date,
    	        'type' => 'numeric',
    	        'compare' => '<='
    	    ),
    	    array(
    	        'key' => 'finish_date',
    	        'value' => $month_start_date,
    	        'type' => 'numeric',
    	        'compare' => '>='
    	    )
    	),
    );

    $events = new WP_Query($args);

Why is this causing a 404 when the year is anything other than the current year?