Fatal Error: Call to member function query() on a non-object

Question

I have the following SQL query,

    if ($conn) {
        $resource = $mysqli->query('SELECT
                    catname 
                FROM 
                    wp_categories
                ');

        while ( $row = $resource->fetch_assoc() ) {
            var_dump($row);
        }

        //var_dump($row);

        } 
    else {
        return false;
    }

However this throws the error: "Call to a member function query() on a non-object".

This is something that appears commonly on here, but I can't see any issues this. Can anyone suggest what the issue might be?

Made and edit to the query.