Flask of Homunculus not working

[ηιgнтмαzє]

So, I read the manual, tried several times to get the co-op working and it doesn't, the flask is used and "nothing happens" is written as effect.
My partner has a controller, I'm playing with a keyboard and a mouse, is it the issue ? Do I have to buy a controller myself just for the sake of playing on local co-op ?????

[Hiboshi]

Each player needs a controller to start the co-op mode, as you can see in the FAQs and the manual (press F1 during the game to open the manual, or search for the PDF file in the game's installation folder).

Regards