Fix - API: `sum()` method should have a default value

api/sum().js

@@ -31,6 +31,6 @@
 jQuery.fn.dataTable.Api.register( 'sum()', function () {
 	return this.flatten().reduce( function ( a, b ) {
 		return (a*1) + (b*1); // cast values in-case they are strings
-	} );
+	}, 0 );
 } );