Black aeras while using weightmaps from tiled landscapes

Hi everyone,

I’m using World Machine Pro and Unreal Engine 4.8 to generate my landscape (I use tiled build and world composition)

So I have three layers used, Grass, Rock and Snow. I exported them from Comprehensive terrain generation like this :

Then for the base color for example I made this material :

But I get thos black areas that looks horrible :frowning:

Any idea ?

Thanks in advance,