Not working, giving an error. How to insert variable into object???

var-into-object-fail.php

function year_sum($year) {
					
   return $minutes_cycled_year = $wpdb->get_var($wpdb->prepare("
   SELECT SUM($wpdb->postmeta.meta_value)
   FROM $wpdb->postmeta, $wpdb->posts
   WHERE $wpdb->postmeta.meta_key='exercise-minutes'
   AND $wpdb->postmeta.post_id=$wpdb->posts.id
   AND year($wpdb->posts.post_date)='$year'
   "));
			
}

echo year_sum(2011);