Ah, thank you both. So yes, more trims. I thought there was only one trim (at the visible split in the top surface), but you need one more. I was trying to cap a similar shape extrusion with these kind of concave parts to the shape and couldn't find an elegant solution as the BlendCap function is only successful with concave profiles to a certain degree. When you have something like this with the 'fingers' it just doesn't know what to do.