NAS-IP-Address = 0.0.0.0

Please excuse me if this has already been covered in the docs or the FAQ (I looked - but nothing jumped out at me). In accounting packets coming from Cisco Catalyst 6513 switches, the NAS-IP-Address = 0.0.0.0. Does anybody know why and if this can be changed? I have tried modifying the aaa accounting commands on the switch, but has not seemed to fix it. On our 3750 series switches, this doesn't happen and the correct switch/NAS ip address is listed in the NAS-IP-Address attribute field. Thanks in advance, Walter