login systeem

<?php
include("connect.php");

$username=$_POST['username'];

$password=$_POST['password'];

$sql="SELECT * FROM admin WHERE username='$username' and password='$password'";

$result=mysql_query($sql);

// This counts to see how many rows were found, there should be no more than 1

$count=mysql_num_rows($result);

// If result matched $username and $password, table row must be 1

if($count==1){

// Register $myusername, and redirect to file "login_success.php"
session_start();
$_SESSION["logged"] = 1;
header("location:admin.php");
}
else {

$_SESSION["logged"] = 0;

echo "geen rechten!";

}

?>
Hier boven zie je mijn login.php script, alleen hij geeft de volgende error

mysql_num_rows(): supplied argument is not a valid MySQL

ik heb de foutmelding gegoogled maar werd er niet veel wijzer van