Can't cast BiUniqueValueRenderer to IBivariateRenderer

[SteveStanton]

I have an instance of IFeatureRenderer that I've determined is an instance of BiUniqueValueRenderer. To pick out the two renderers involved, I then tried to cast it to IBivariateRenderer, but it says it doesn't implement it. How come?

I'm on 10.0, working in C#

[PeterYurkosky1]

We'll need a code sample. BiUniqueValueRenderer definitely implements IBivariateRenderer, so this should cast just fine.