Laravel unit testing assertRedirectedTo

I am working with some unit testing and i have difficult with one kind of assertions

  $this->assertRedirectedTo

Here is my very basic example controller code

        // example.com/form
        public function postForm()
        {
            $data = "my data";
            $this->mylogic($data);
        }
        private function mylogic($data){
            // operations with $data ... after some logics
            return redirect()
                ->route('Gracias')
                ->send();
        }

And this is my very basic example of my unit testing

        public function REDRIRECT_TEST()
        {
            $this->call('POST','form'); // it works
            $this->assertRedirectedTo('gracias'); // it fails
        }

and i get this error

        Failed asserting that Illuminate\Http\Response Object (...) is an instance of class "Illuminate\Http\RedirectResponse".

This error happen when i use redirect inside another method

    private function mylogic($data){
        return redirect()
            ->route('Gracias')
            ->send();
    }

Any idea how to deal with assertRedirectedTo where the redirect() is inside another method?

Thanks.