--- /wp-includes/query.php	2016-10-19 16:02:27.728266004 +0200
+++ /wp-includes/query.php	2016-10-22 10:00:49.000000000 +0200
@@ -2333,7 +2333,21 @@
 		$primary_meta_query = false;
 		$meta_clauses = $this->meta_query->get_clauses();
 		if ( ! empty( $meta_clauses ) ) {
-			$primary_meta_query = reset( $meta_clauses );
+
+			/* start : patch - find the primary_meta_query by 'orderby key' */
+
+			$primary_meta_query = false;
+			foreach( $meta_clauses as $meta_clause ) {
+				if ( $meta_clause['key'] == $orderby ) {
+					$primary_meta_query = $meta_clause;
+					break;
+				}
+			}
+			if( ! $primary_meta_query ) {
+				$primary_meta_query = reset( $meta_clauses );
+			}
+
+			/* end : patch */
 
 			if ( ! empty( $primary_meta_query['key'] ) ) {
 				$primary_meta_key = $primary_meta_query['key'];