Autoloading classes

Hi there, I’ve included classes.php in my core plugin file.
In classes.php I try to autoload all my classes. When I activate the plugin it should autoload them, but instead I’m getting an error that it tries to autoload a theme .class.php, which isn’t even in my classes folder.

spl_autoload_register( function($classname) {

    $class      = str_replace( '\\', DIRECTORY_SEPARATOR, strtolower($classname) ); 
    $classpath  = plugin_dir_path(__FILE__) .  DIRECTORY_SEPARATOR . 'classes' . DIRECTORY_SEPARATOR . $class . '.php';
    
    if ( file_exists( $classpath) ) {
        include_once $classpath;
    }
   
});