Выдает Fatal error: Uncaught Error: Call to a member function fetch_assoc() on boolean in D:\OpenServer\domains\cars\cars\login.php

выдает ошибку Fatal error: Uncaught Error: Call to a member function fetch_assoc() on boolean in D:\OpenServer\domains\cars\cars\login.php:7 Stack trace: #0 {main} thrown in D:\OpenServer\domains\cars\cars\login.php on line 7

Сам код:

<?php include "inc-files/before_content.php";

$mysqli = new mysqli('localhost', 'root', '', `tbl_users`);
$mysqli->set_charset('utf8');
$result = $mysqli->query("SELECT * FROM tbl_users WHERE username='" . $_POST["username"] . "' and password='" . $_POST["password"] . "'");
$mysqli->close();
if($row = $result->fetch_assoc())

{
    $_SESSION["username"]=htmlspecialchars(stripslashes($row["username"]));
    $_SESSION["usertype"]=htmlspecialchars(stripslashes($row["usertype"]));
    $_SESSION["personname"]=htmlspecialchars(stripslashes($row["personname"]));

}
else echo "<span class='errMag'> Неверный логин или пароль.</span>";
header("Location: .");
exit;
?>
<?php include "inc-files/after_content.php"; ?>