Are x:Bind functions supposed to do null checks in the path?

[amtopel]

From what I can gather, if you x:Bind to a chain of properties, the framework will check for null values at each step and won't carry on if it encounters a null value.

But if you x:Bind to a chain of properties with a function as the leaf step, and there's any null value along the chain, the application will crash with an access violation.

Is that the expected behavior or a bug? Thanks for any thoughts or input. And just to clarify, my question is about null checks for the path, not the function argument.