This problem still stumps me. I was able to extrude a spiral object when I had a zig-zag forming in the middle (though only with every other zig-zag. for examples, when there were 2, 4 and 6 spline edges crossing the distance from the start point to the end point, it was a malformed spline. Then, when I had the zig-zag and there was 3, 5, and 7 edges in-between it extruded. Yet then it didn't seem to work when I did this with curved splines, and I found it was the opposite (2, 4, and 6 switched with 3, 5, and 7.)

Now I find that it doesn't work with more than one loop of the spiral though... so I'm still stumped.

Reading this over I'm completely sure I've confused everyone here, but don't worry, I'm as confused as you are.