Nullable类型抛出错误“必须是异常的实例”

Question

After adding ?string type to the function paramerers it throws me an error.

Error:

Type error: Argument 1 passed to Whoops\Exception\Inspector::__construct() must be an instance of Exception, instance of ParseError given, called in /var/www/website/vendor/filp/whoops/src/Whoops/Exception/Inspector.php on line 80

Code:

public function getPayments(?string $searchKey = '', $searchByColumn = "id")
{
     return $this->model->where($searchByColumn, '=', $searchKey)->get();
}

I am using laravel4 with php7. What can I do about it?