Crazy behavior of my new Compositor Node

Hi everyone!

I am currently working on a little project and implemented a new compositor node in Blender.
But when using it in the compositor, in the moment I plug it in, the preview disappears… When hitting F12, the node works as expected.
This is strange enough but then I figured out, that when I plug in the Image to the factor Socket of my Node, the preview again shows up.
How could that be? Does anyone has an idea where the problem might be?

Thanks for your help!