Notice: Trying to get property 'num_rows' of non-object in C:\xampp\htdocs\Project2\verify.php on line 10

<?php
if (isset($_GET['vkey'])){
    //verification process
    $vkey = $_GET['vkey'];

    $mysqli = NEW MySQLi('localhost','root','','bookfly');

    $resultSet = $mysqli->query("SELECT verified, vkey FROM signuptable WHERE verified =0 and key = '$vkey'LIMIT 1");

    if($resultSet->num_rows == 1){
        //validated the email
        $update = $mysqli->query("UPDATE signuptable SET verified = 1 WHERE vkey='$vkey' LIMIT 1");
        if($update){
            echo "Your Account has been verified . You may login now.";
        }
        else{
            echo "$mysqli->error";
        }
    }
    else{
        echo "This account is invalid or already verified ";
    }
}
else
{
    die("Something Went Wrong");
}
?>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>

</body>
</html>