When is "cast failed" called on "Cast to..."

Hi, I have a blue print animation and in the Event Graph I’m casting another blueprint for some IK calculation (tryed to copy from the examples).

It works only when the IK blueprint has Character.h as parent. But when I change it to another c++ class, like the auto generated file for a playable first person character in C++ “prototype1character.h”. The animation blueprint fails to cast it.

Any suggestion? Or more generic question, when a cast fails?
Thank you and sorry for my english.

+1 I can’t cast to my player when trying to do IK either.