MySQL, Searching for a nonexistant record

this is a piece of a module detection script for a dynamic content engine i am making.

If a module is in the folder, it adds it to the database when the admin tool is called.

It checks to see if the module is already in the database, and if it is in the folder, but not in the database, then it adds it to the database

the problem is where it says if(!mysql_num_rows($check)) { It functions perfectly fine, but MySQL outputs an error when it does not find anythign in the database and says that the result from mysql_num_rows() is invalid

thanks a lot if you can help XD

<?php
     //search for uninstalled modules
     $curdir = opendir("modules");
     include($connect_src);
     while($dir = readdir($curdir)) {
          if(strlen($dir) >= 4){
                $dirstr = $dir[0].$dir[1].$dir[2].$dir[3];
               if(is_dir("modules/".$dir) && ($dirstr == "dce_") && is_file("modules/".$dir."/info.php")) {
                    $query = "SELECT dir_name FROM modules WHERE dir_name = '".$dir."'";
                    $check = mysql_query($query);
                    if(!mysql_num_rows($check)) {
                         include("modules/".$dir."/info.php");
                         $query = "INSERT INTO modules VALUES ('NULL','".$name."','".$dir."','".$root."modules/".$dir."','".$version."','".$vers_output."','".$creator."',NOW())";
                         register_module($connect_src,$query);
                    }
               }
          }
     }
?>