PHP Warning: mysqli_num_rows() expects parameter 1 to be mysqli_result, boolean given in

<?
	session_start();
	if(isset($_SESSION['administrator'])!="")
	{
		header("Location: accounting-home.php");
	}
	include_once 'dbconnect.php';
	include("timeout.php");

if (isset($_POST['login'])) {
	
	$acc_username = mysqli_real_escape_string($con, $_POST['acc_username']);
	$acc_password = mysqli_real_escape_string($con, $_POST['acc_password']);
	$result = mysqli_query($con, "SELECT * FROM cpemis_accounting WHERE acc_username = '".$acc_username."'") or die(mysqli_error());
	$row = mysqli_fetch_array($result);
		
	if($row['acc_password']==md5($acc_password))
	{
		$_SESSION['administrator'] = $row['acc_id'];
		$_SESSION['username'] = $row['acc_name'];
		$_SESSION['userpass'] = $row['acc_password'];
		$_SESSION['loggedin_time'] = time();
		header("Location: accounting-home.php");
	} else {
		$errormsg = "Incorrect User and Password or Account Disallow on system.";
	}
	
	if(isset($_SESSION["administrator"]))
	{
		if(!isLoginSessionExpired())
		{
		header("Location:accounting-home.php");
		} 
		else
		{
		header("Location:logout.php?session_expired=1");
		}
	}
	if(isset($_GET["session_expired"]))
	{
		$message = "Login Session is Expired. Please Login Again";
	}
}
?>