DI-614+ & DHCP to unknown MAC Address

I have a D-Link DI-614+ Wireless router/firewall. It is configured to
be a DHCP Server. It is able to serve an address to the hard-wired
computer that is connected to one of the switch ports.
It also decides to serve the other 99 available IP Address to an
unknown MAC Address. It's the same Address every time:
52-41-53-20-00-C0. I have WEP enabled, and know that I am not getting
hijacked; this has been happening since day one of the installation,
and even when I disable the wireless connection completely.
I've tried upgrading the Firmware, with no luck.
HELP!

-Tim