How to convert a returned value to text in Report Builder 3.0?

I am creating a custom TFS report from the analysis cube using Report Builder 3.0 and trying to figure out a way to do something.

I have a boolean field that either returns "Unknown" if blank or "-1" if checked. So when I run the report, I either see Unknown if the field is not checked (checkbox) or -1 if the field is checked if that makes sense.

What I want to do is use an expression to return "Yes" if the field returns "-1". So I am trying to convert the original returned value of "-1" to Yes.

I'm not worried about the unknown but it would also be nice to display unknown as just a blank value. 

In summary, I want to display "Yes" for any returned values that = "-1" and display "just a blank field or maybe no" for any returned values of "unknown".

I tried to use the following expression "

=IIf(Fields!FieldName.Value =-1, "Yes", "No") 

This expression only returns the value "#ERROR" so obviously that won't work. Can someone tell me how to what I need?

Thanks in advance for any help!