Lofting with splines doesn’t work

I am trying to draw tapered ridged waveguide by using a loft.
idraw the shape at each end and add guide lines. If I make all the
guide lines straignht lines everything is fine. However two of the
linesa needt o be curved. When I replace twwo of the straight ;lines
with spline curves the loft fails. It seems intuitive that the
loft should follow whahtever curve us provided.



The question is how to I do a loft where a guide curve is spline?



Thanks,



Jim