error: you cannot delete an item with 3 edges

[mudplugga]

I’ve manually adjusted an autobuilt gable end roof over a rear exterior knee wall in cross section view in CAD mode. But I can’t delete the last section of the old line. An error message comes up saying “11643 you cannot delete an item with 3 edges”. But it’s got 4 edges, merging the handles doesn’t work in CAD mode, the delete keyboard key ineffective and the Home Designer delete button is greyed out with CAD off. This bit of extraneous CAD doesn’t show up in the floor plan. Deleting the wall in floor plan view deletes the whole wall and when I rebuild it, its gable end cuts through the roof line above the knee wall as the original did so I’m back to square one. I ticked knee wall in the rear wall dbx and the rest of this roof plane works fine over it. Any suggestions warmly welcomed. If I’ve got to live with this, so be it, its still better than my manual roof planes.
This forum is really useful, as a new user the learning curve is steeper than some of the spikes in my first attempts at terrain modelling.