mysqli - PHP Fatal error: Call to a member function bind_param() on a non-object

The bind_param that is failing is this

$stmt->bind_param('ssss', $this->template->forms->reg_username, $this->template->forms->reg_password, $this->template->forms->reg_email, "Test");

the prepared statement is

$stmt = $this->mysqli->prepare("INSERT INTO 'users' (username, password, email, referr) VALUES (?, ?, ?, ?)");

The whole Register function is

public function Register() {

    unset($this->template->forms->error);
    $this->template->forms->setData();

    if(isset($this->template->forms->submit)) {

        if(!isset($this->template->forms->reg_username)) {
            $this->template->forms->error = "You must enter an username";
        }


        if(!isset($this->template->forms->reg_password)) {
            $this->template->forms->error = "You must enter a password";
        }

        if(!isset($this->template->forms->reg_password_confirm)) {
            $this->template->forms->error = "You must confirm your password";
        }

        if(!isset($this->template->forms->reg_email)) {

            $this->template->forms->error = "You must enter an email address";
        }

        if($this->template->forms->reg_password != $this->template->forms->reg_password_confirm) {
            $this->template->forms->error = "Your passwords must match";
        } else if(strlen($this->template->forms->reg_password) <= 5) {
            $this->template->forms->error = "Your password must be 6 characters or more.";
        }

        if(!$this->template->forms->error) {

            $stmt = $this->mysqli->prepare("INSERT INTO 'users' (username, password, email, referr) VALUES (?, ?, ?, ?)");
            $stmt->bind_param('ssss', $this->template->forms->reg_username, $this->template->forms->reg_password, $this->template->forms->reg_email, "Adam");
            $stmt->execute();
        }

    }
}