Fatal error: Unsupported operand types

Hi to all,
when I go to BWS plugins->portfolio to set up my preferencies and I click on “save” even if I don’t change anything, I get this error:

Fatal error: Unsupported operand types in C:\xampp\htdocs\creazioni-raffa\wp-content\plugins\portfolio\portfolio.php on line 393

the code is the following:

// Create custom permalinks for portfolio post type
if( ! function_exists( 'prtfl_custom_permalinks' ) ) {
	function prtfl_custom_permalinks( $rules ) {
 	$newrules = array();
		$newrules['portfolio/page/([^/]+)/?$'] = 'index.php?pagename=portfolio&paged=$matches[1]';
		$newrules['portfolio/page/([^/]+)?$'] = 'index.php?pagename=portfolio&paged=$matches[1]';
  return $newrules + $rules;
	}
}

And the error occurs on the “return $newrules + $rules” row.

How could it be fixed?

Thanks

https://www.ads-software.com/extend/plugins/portfolio/