So I have my own minecraft server that I created using the downloads that you can get from minecraft.net. I've been able to connect to it just fine using my minecraft.exe, and others have been able to connect to it as well. Although, whenever I try to connect using the World of Minecraft client, all I get is this black screen, and then the program stops responding. I can join other servers using the World of Minecraft client, and I do have the right information put into the url bar. On my server I get something like this whenever I try to connect:

Disconnecting /MY.IP.IS.HERE:PORT: Protocol error
/MY.IP.IS.HERE:PORT lost connection

Any tips on how I can get this to work?