ByFunction defined nodes in user generated nodes error

I'm trying to create a user generated node which takes a list of points and draws a path of cylinders through the points, filleted at the corners. Everything works as expected when all the nodes are separate, but when i combine them into a GNT, i get an "Internal error, Objects reference not set to an instance of an object". I created GNTs of just a portion of the nodes, and found that it works until i include a node thats defined using ByFunction. This stripped down GNT takes the list of points, draws lines through them using a line node defined with ByChaining, and then an Arc node uses a function to fillet each successive pair of lines. I don't see a way to achieve this without a function as the regular FilletTangentToCurves can only fillet two lines. Is there any way to get a ByFunction node to work in a GNT?