Push dim to base part - "cannot assign"

Using Creo 3 M100

I've got an inheritance part where I'm trying to control a dimension in the "base" part with a parameter in the "master" part.  In other words, if I change a parameter value, then the geometry changes, even though the parameter and dimension are in different models.

I took this screen shot in Wildfire 5 to remind me how to write the relation.  I'm applying this technique to new parts in Creo 3, but the Relations editor gives me an error of "Cannot assign to a part-driven value".

Am I missing anything, or is this a bug in Creo 3?