Has the navigation mesh limitations ? or bugs ?. It often becomes black.

i often endup with a black not working navigation mesh.

Scenario, i want to let a cube follow a spiral, as i want a smooth spiral.

i added extra object types to blender, then you get a spiral nurbs.
I make my spiral tell the nurbs to put z up on all verticles.

I create a face or cube, apply array modifier 500 times to it, then a curve modifier (selecting the spiral)
I turn the faze in edit mode 90 degrees (so Z stays upwards).
I aply array (with merging), and i apply curve.

Resulting in a spiral of 500 faces.
I tell the game engine that this is a navigation mesh, navigation mesh turns black and isnt working.
Sometimes when creating simple logic for cube to follow this black navigation mesh blender even crashes (it just did).

On rare ocasions it works but i dont understand what is the main cause for black navigation meshes (bug ?)
Using latest Blender, or official blender doesnt make a difference.

I think only mesh objects are supported,