OffsetCrv on Surface failure - Rhino v6

In the attached file are 2 surfaces with curves on them. I need to offset the green curves through the points represented by the white lines that hit the edge of the surface. This works perfectly on the upper surface as shown by the red lines - and fails, always, on the lower surface. I can’t find the difference between the two situations.

Any thoughts are welcome.

RobbOffsetFail.3dm (331.7 KB)

Hm. It appears to fail to the inside - that is, where the offset curve needs to be extended to run off the edge.

Still poking…