GetRawInputData always returns null as device handle regardless of input type

Supposedly, it was only supposed to happen with high-precision trackpads, but it happens to all my devices. I do however have a graphic tablet and an old PS/2 keyboard connected to my motherboard, but all devices cause such issues on my PC at least. I tried my best to follow MS documentations as closely as possible, so unless there was something missing from there (I've read something about OnInputEvent.Post(), but it might be related to some other API) I don't think I've done something wrong.