HMVC - Extend MX_Controller gives me an error, CI_Controller do not!?

[eluser]Sven Delle[/eluser]
I read that I should extend with MX_Controller if I use HMVC for codeigniter. But I've never managed to ever extend that. I've always used CI_Controller.

Now, can anyone explain to me why that is - and what's the point of saying I should use MX_Controller when I can't?