filemtime() warning when enqueuing style within plugin

I’m receiving the following filemtime() warning:

Warning: filemtime(): stat failed for
https://testing.local/wp-content/plugins/test-social-icons/css/style.css
in
/app/public/wp-content/plugins/zoo-social-icons/zoo-social-icons.php
on line 29

Here’s how I’m enqueuing the stylesheet within my plugin:

function test_styles_scripts() {
   $dir = plugin_dir_url(__FILE__);
   wp_enqueue_style( 'test-style', $dir . 'css/style.css', array(), filemtime( $dir . 'css/style.css' ) );
}
add_action( 'wp_enqueue_scripts', 'test_styles_scripts' );

The stylesheet is correctly enqueuing. Why am I getting this warning?