Question about Separator transformations.

<%# (Eval("NodeAliasPath").ToString().Contains("Products")) ? "</ul>" : "" %>

Is there a reason why this would not work as a separator transformation? I am getting the following error:

Message: [CMSAbstractTransformation.DataBind]: Object reference not set to an instance of an object.

Using hierarchical transformations to generate a site navigation. I had a specific need to alter the behavior for a particular section. Just trying to clean up some extra HTML on the items that do not need to behave this way and when I tried using the above code for the separator, the error occurred.

Just wondering if there is a specific reason why this would occur, or if my error is elsewhere.

Thank You