Users in my social network can't register and login

I made a simple social network system, with register and login function, but whem the user register and try to login, he give this message:

Fatal error: Call to a member function rowcount() on a non-object in home/login.php on line 7

There is some problem in my login.php code?:

<?php
include('dbcon.php');
$username = $_POST['username'];
$password = $_POST['password'];

$query = $conn->query("select * from database where username = '$username' and password = '$password'");
$count = $query->rowcount();
$row = $query->fetch();
if ($count > 0){
session_start();
$_SESSION['id'] = $row['member_id'];
 header('location:home.php'); 
}else{
 header('location:index.php'); 
}
?>