Expression IIF ISNOTHING display "#Error"

Hi All,

I am using this expression to check if there is a value in this field, but it displaying "#Error" in the report. When I run the query in SQL the value is null. Am I doing something wrong?  I have used this type of expression before, but the only thing different is the values are name1, name2, name 3.

=iif(isnothing(Fields!Care_Team_Assigned.Value),"a", Fields!Care_Team_Assigned.Value.ToString().Replace(",",vbCrLf))