SQL RDL IIF condition to check for null

Question

I want to display "X" if the field has a certain text, and "" if not.

I have tried this but shows #Error when field is null:

=IIF(Fields!ReasonNonCompliance.Value.ToString().Contains("Assessment Personnel;"),"X","")

I have tried this with the same #Error.

=IIF(IsNothing(Fields!Reason.Value),"",IIF(Fields!Reason.Value.ToString().Contains("Reason Assessment;"),"X",""))

Any ideas?