Linksys Range Expander causing IP conflicts?

I tried adding a Linksys WRE54G range expander to my home wireless network. My router is a Linksys WRT54GS. My Internet provider is Comcast. I have 3 PCs, a Nintendo Wii and an Xbox 360 that are all connected to my network.

While trying to "connect" the range expander to the network, is started encounter the following error message "System Error- There is an ip address conflict with another system on the network ". When I look at the table in the router that lists the assigned IP addresses, I see no conflicts. Why would I be getting this error message and what can I do to resolve it.

I'm almost ready to box the range expander back up and bring it back to the store.