I tried leaving off the IP address and I get the same error. I was actually entering the command at the console of the PanelDue. I doubt it's relevant, but the end of the IP address runs off of the text entry space to the right and out of sight, and since it's not echoed to the screen I can't verify that the entire command is in fact being sent.
I just connected to it via USB and sent the same command in TeraTerm and it works fine.