刷新 token 方法无法返回 Auth::guard ('API')->user () 当前用户吗？

代码报错，求解。。 "message": "get_class() expects parameter 1 to be object, null given",

public function update()
{
    $token = Auth::guard('api')->refresh();
    return $this->respondWithToken($token);
}
    public function respondWithToken($token)
{
    return $this->response->item(Auth::guard('api')->user(), new UserTransformer())
        ->setMeta([
            'access_token' => $token,
            'token_type' => 'Bearer',
            'expires_in' => Auth::guard('api')->factory()->getTTL() * 60,
        ]);
}