Error creating custom variable

I’m trying to create custom variable (%cars%).

My code is…

add_action('rank_math/vars/register_extra_replacements', function() {

	rank_math_register_var_replacement(
		array(
			'variable' => 'cars'
		), array(
			'name' => 'Total cars',
			'description' => 'Total cars',
			'example' => count_cars()
		), 'count_cars');

});

I’m getting this error:

Warning: preg_match() expects parameter 2 to be string, array given in /var/www/vhosts/cars.com/httpdocs/cars.com/wp-content/plugins/seo-by-rank-math/includes/replace-variables/class-manager.php on line 108

Ok, I know first parameter of function is string

rank_math_register_var_replacement(string, array, string);

But, if I define first variable like a string, I get an error:

Fatal error: Uncaught InvalidArgumentException: $variable is required for the cars variable.