Hi Rich_Art - basically by only selecting just that edge there and not the one above it, you're trying to tell MoI to build a fillet that would result like this:



It doesn't like to do stuff like that which would cause an open gap to be created and disrupt the solid.

So to get that kind of thing filleted you need to also select the little edge above there, and also the similar one at the bottom:



Then that can be filleted without your solid getting torn open:




But usually it's best in things like you are showing there to have those edges be filleted all at the same time as the other ones instead of separately like this. Except when you're doing different radius values you can do it separately with the largest radius ones going on first and then smaller radius ones after that.

But you probably want to do something like that all at once which will result in a different style corner patch at that kind of juncture.

- Michael