Hi Gary,
the # symbol doesn't only represent null values, it represents all disallowed charachters in the SAP application. My advise would be to trace the value in the PSA and try to see the value in the source flatfile afterwards.
You can enter these symbols/values/charachters in transaction RSKC to add them to the list of permitted characters, see the following note...
1373402 - Issues with invalid characters or SID Generation
Best of luck,
Colm