Hi Mauro - the way curve filleting works is that each curve segment is trimmed by the arc pieces that are generated.

In your particular case there the original curve segment would be trimmed away to nothing at all, so the code that tries to do the trimming gets an error in that case and that's why it does not work.

It should probably not be too difficult to fix up, it just needs a special case added for this particular circumstance. I can change it to just throw out the original curve piece in this case rather than trying to trim it. I've added it to my list of stuff to fix.

In the meantime the easiest workaround is to just draw a circle in a case like this and just delete the square rather than using fillet - the result you are looking for is a full circle, right?

Thanks for reporting it!

- Michael