When attempting to fillet a polyline, a response "Cannot fillet polyline arc and line segments." is returned, or an unexpected result is returned.  I've attached a file with three examples.  On the left, the fillet is shown as expected.  In the middle, an unexpected result is returned when filleting.  On the right, the error message is returned.  The difference between the three is this.  The left was filleted on entities not part of a polyline.  The middle is two polylines that do not fillet correctly.  The right is a single polyline (of curves and lines) that returns the error.  Any help, or improvement on the software would be appreciated.  I commonly fillet polylines in these ways.  Thanks!