comments_open FUNCTION

Hello.
I believe this function is missing something…
Wouldn’t be good a check on $_post or $post_id ?

function comments_open( $post_id = null ) {
  if($post_id ){
	$_post = get_post($post_id);
     if($_post){
	$open = ( 'open' == $_post->comment_status );
	return apply_filters( 'comments_open', $open, $post_id );
     }
  }
}

I say this because i get a strange error when calling comments_open() with no parameter:

Notice: Trying to get property of non-object in /home/hutanuro/public_html/catalin/wp-includes/comment-template.php on line 776