AutoCad 2D :: 2012 Not Closing Lines?

I am completely new to AutoCAD. I have been given a 2D DXF file with the design of an ion source. I have removed large parts of the design as I only need to electrode shapes.

I need to import the electrodes as a 2D DXF into my code to run simulations with them. However...

The code can read layers, and it searches for a closed loop within a layer and uses that to shape its electrodes. So I have isolated each electrode and saved them.The issue I'm having is the code finds no closed loops. Further investigation let me to the close, fillet, and boundary commands. Which I assumed would do the trick. However after a number of hours I am no closer to my goal.

I have isolated the problem to be linking a semi-circle to 2 lines. Because if I remove the semi-circle and replace with a straight line code will happily detect a closed loop. I have converted the entire thing into 1 polyline, and then to make sure its closed I have used the boundary command to create a polyline outline of the shape.

Even then, it is apparently not a closed loop. I was wondering if there is a glaringly obvious mistake I am making here, or if it comes down to resolution within the code. I'm looking into part 2