Bagaimana mengatasi warning mysqli_num_row

Bagaimana mengatasi Warning: mysqli_num_rows() expects parameter 1 to be mysqli_result, boolean given/public_html/login.php on line 11

Pada coding yang telah di hosting di 000webhost.

<?php
session_start();
include 'koneksi.php';

if (isset($_POST["login"])) {

$username= $_POST["username"];
$password= md5($_POST["password"]);

$sql= mysqli_query($koneksi, "SELECT * FROM user WHERE username='$username' AND password='$password' ");
$cek= mysqli_num_rows($sql);

//cek apakah user terdaftar atau belum
	if ($cek > 0 ) {
		if ($data= mysqli_fetch_assoc($sql)) {

			//jika yg login admin
			if ($data['level'] == "admin") {
				$_SESSION["username"]= $username;
				$_SESSION["level"]= "admin";
				$_SESSION['status']= "admin";

				echo "<script>alert('Anda masuk sebagai admin');
   				document.location.href='data/index2.php';</script>";

			//jika user biasa
			}elseif ($data['level'] == "user") {
				$_SESSION["username"]= $username;
				$_SESSION["level"]= "user";
				$_SESSION['status']= "admin";

				echo "<script> alert('Anda masuk sebagai user biasa');
   				 document.location.href='index.php';</script>";

			} else{
				echo "
					header('location:login.php');
				";
			}
		}
	}
}

?>