Cognos Error RSV-VAL-0032

This article explain how I got Cognos Error RSV-VAL-0032 and solved it.
In a report I have a filter on Date From. It works with a DatePicker Prompt. Often the client wants to print the report or view in PDF. When the report is started and no Date From is chosen, the parameter p_date_from is empty. So when I show this parameter in the report it is empty.
To solve that I created a report expression with the following case:

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