User-Name missing

Hi all! I execute an external script as soon as I get an Access- or an Accounting-Request. The scripts collects (from the environment variables) all the attributes received and writes them to a Database. I encountered a problem recently. The attribute User-Name is missing (its value equals NULL) for the Accounting-Requests but it is there in the Access-Requests. I find this strange that the User-Name attribute is sent with certain package types and not with others. My server is a freeRADIUS 1.0.3. Thanks for any contributions to solving this problem! -- Vicky