Problem joining an Arc curve

Hi, I’m a bigginer in Grasshoper, and had get to this problem that I can’t fix and haven’t found a solution.

I built an arc between 3 points, two of are allready used in a joined structure. When I join this arc to the rest of the curves, I’m not able to convert it to a surface, neither extrude it.

Does anyone knows wy this could be happening?

Thanks! (14.1 KB) (14.1 KB)

You’ve got to Flatten the input to Join Curves but you still get two curves, a closed one and the line you also are inputting. I imagine the final extrusion is what you want, so I don’t see what purpose that line serves.

It worked!