Task Status updates through PSI

Hey,

I'm still stuck with the statusing.updatestatus() method. 

I have a task, that could be assigned, but does not have to be. Id like to change the start date as well as physical work completed in percent for that task through PSI

I can create the the changexml and its valid against the changelist scheme (using the changexml tool of project sdk 2007). Now my problem is that I get a Fault Exception saying that Column 'ASSN_UID' does not accept null values. 

Now my question: Why are assignment uid's in the game? I just wanted to update that task, not the assignment. Also, how is it possible to approve this statusupdate automatically (or is this the usual way via SubmitStatus(), UpdateStatusApprovals() and QueueApplyStatusApprovals() ?)

Thanks for your help, 

Kind Regards, 

Tom