Invalid curves in FaceB component

Hi :)

I am new to Grasshopper (trying out various definitions) and i have stumbled upon this problem.

FaceB component returns some Invalid Curves. And it is causing problems in the MLoft, Merge and wbJoin components (well, i think it does, maybe there is another problem?)

I have tested other surfaces, Cone and Cylinder are working perfect. But with a sphere or any custom surfaces, i get this error.

Can you point me in the right direction? Or how to fix this? And if its not a problem, could you pls where am i making a mistake? So i can be better at it :)

PS: i tested some components with the intention of a workaround, but with no luck.

Thank you very much :)