How do i use Isosurface by FFT-Function?

I have a 100x100x100 Matrix, and i need to use FFTN on it, then plot the FFTN-funkction with isosurface.

A is a 100x100x100 Matrix. When i do isosurfe(abs(fftn(A))), im getting a wired triangle thing.

How should i do it correctly?

Many thx.