mysqli_num_rows returning 0 always

 if(!empty($_POST['login'])){
        $email=mysqli_real_escape_string($conn,$_POST['email']);
        $password=mysqli_real_escape_string($conn,sha1($_POST['password']));

        $query=mysqli_query($conn,"SELECT * from user WHERE email='$email' and password='$password'") or die("Failed query");
        $rows=mysqli_num_rows($query);
        mysqli_store_result($conn);
        $data=mysqli_fetch_assoc($query);

When I put $rows in a if condition, it always return 0. Please tell me what's wrong