Retrieve data from MySQLi under prepared statement

Question

I have this MYSQLi query below. The connection to the database is no problem. I tried to use bind_result too, it could retrieve the data without any problem. However when I use select * statement, it returned the result only True or False. It didn't return any data. Is there anything wrong?

Error: returned from PHP

Fatal error: Call to a member function fetch_array() on boolean in line 20

Code:

$conn       = new CDatabase();
$mysqli     = $conn->dbConnect();

$sqlstmt = "SELECT * FROM general_staff WHERE generalstaff_userid =? AND generalstaff_password =? LIMIT 1";

if ($mysqli->connect_error) { echo "Connection failure."; }

    if($stmt = $mysqli->prepare($sqlstmt)){
        $stmt->bind_param("ss", $username, $password);
        $stmt->execute();
        $stmt->store_result();
        $stmt_result = $stmt->get_result();
        $stmt_rows = $stmt->num_rows();

        if ($stmt_rows >0) {    

            while ($row = $stmt_result->fetch_array()){
                echo $row[1];
        }
    }
?>

By adding this code below to get error code, it returned 1193

if (!$mysqli->query("SET a=1")) {
printf("Errorcode: %d\n", $mysqli->errno);