Black preview on a MaterialFunction when using another MaterialFunction inside

Hi,

I have a lot of MaterialFunctions. The basic one, with no dependencies, are fine, preview OK. Materials that use these MTFs are fine as well with preview.

But if I make a MTF that uses another MTF, the preview for this MTF is broken. The material is black in the content browser and in the preview pane. But it display correctly ingame and in a material using it.

Any idea why ?

Thanks