RDL-Report show timestamps with suffix suggesting summertime/ wintertime

Question

In order to achieve a clear differentiation of time stamps I want to ad a suffix suggesting if its daylight saving time.

Everything is realized in tables of a rdl-report that gets the datasets from a visualization. To differentiate between the two options the suffix should either be called "SZ" (summertime) or "WZ" (wintertime).

Table in the RDL-Designer

The Dataset is displayed in a Table with following function for the Timestamps "Zeit Gekommen/Gegangen/Quittiert"

=IIf(IsNothing(Fields!TIMEGOES.Value),"KeineZeit",IIf(Fields!TIMEGOES.Value.IsDaylightSavingTime(),Fields!TIMEGOES.Value + " (SZ)",Fields!TIMEGOES.Value + " (WZ)"))

As long as there is a valid timestamp it is displayed correctly. (DD.MM.YY hh:mm:ss (SZ/WZ)). If the timestamp is empty a error occurs.

Table with error

To check if my statement to check für empty values has issues I've already tried:

=IIf(IsNothing(Fields!TIMEGOES.Value),"Empty","Value")

Without the second if statement it works just fine.

So there has to be a problem with the statement as soon as nested statement is added.

Is there a problem with the datatype of the table?