Fatal error: Call to a member function fetch_array() on a non-object mysql

Question

Im getting this error when I call my php file. I want to fetch the row returned by the query. It stores basic info like name and surname. I expected to resultValue to have the following structure after the second commented line but I get an error instead:

$returnValue["name"] = "jack"

$returnValue["surname"] = "london"

I know there is similar questions and Im sure I looked into all of them but I still have the same problem. Im sure that my procedure works because I ran it on the server and It returned the row I expected.

<?php
require("Connection.php");

$userName = htmlentities($_POST['userName']);
$password = htmlentities($_POST['password']);

$conn = new mysqli(Connection::$dbhost,Connection::$dbuser,Connection::$dbpass,Connection::$dbname);
$returnValue = array();

try{
    $stmt = $conn->prepare("CALL login(?,?,@res);");
    //stored procedure returns a row containing user information

    $stmt->bind_param('ss', $userName, $password);
    $res = $stmt->execute();

    $returnValue = $res->fetch_array(MYSQLI_ASSOC);//Error on this line

}catch(Exception $e){
    $returnValue["result"] = 0;
}

echo json_encode($returnValue);
$conn->close();
?>