I guess your talking about the t-spline surface body you made?
For reasons that are to difficult to explain here, tspline bodies don't always play well with the surface and solid tools. Particularly around star points and creased edges. or other funkiness like you have here. so work arounds have to be employed.
Here's a screen cast of one possibility.
I projected the edge of the surface into a sketch, then I offset that sketch to the inside just a little bit. Then extruded the resulting profile up to the surface.
