Using an expression to set the style of label font on a category axis in Report Builder

Question

I am trying to set the bold style of a category axis label given a certain label value. I have added an expression to the style attribute of the label font in the category axis properties window. The expression doesn't evaluate. Here is an example of the expression I am trying to use:

=IIF(Fields!FieldName.Value = "SomeValue", "Heavy","Default")

If i switch the heavy and default in the above code then all the labels are set to a heavy bold. I can't figure out why it doesn't evaluate correctly.

I am using SSAS 2008 and SSRS 2008.