Exception when Setting Alphamaps for TerrainData

Hey everyone,

i am currently working on some procedural heightmap generation for the unity terrain system. Now i want to apply Texture layers depending on the height to create biomes. I roughly followed this example:
https://docs.unity3d.com/ScriptReference/TerrainData.SetAlphamaps.html
I can blend two Layers together perfectly fine, however when i want to add a third layer I get this exception:

I changed the third float array value accordingly to 3 but it still throws that exception.
Are there only two Layers allowed or am i missing something else ?
How can I blend more than two TerrainLayers together via skript ?

I am using Unity 2018.3 and the Lightweight Render Pipeline.

Cheers

Lucas

 

Update: When I add a new layer via the Terrain Component it works fine with the script.