mysqli_num_rows() expects parameter 1 to be mysqli_result,

$conn=mysqli_connect('localhost',$username,$pass,$db);
        $query="SELECT * FROM account WHERE username='$nim' and password='$pwd'";
        $result = mysqli_query($conn,$query);
        $count = mysqli_num_rows($result);
        if ($count!=0){
            return true;
        } else {
            die ('No such account found!');
            return false;
        }