This is normal behavior on the ICX.
In order to send a shell command, the ICX specifically requires you to spawn a shell, i.e a normal login followed by the command you want to run.
Otherwise, the ICX supports 'scp' using the remote command functionality, and if you try to use the remote command function with a different command than scp, it generates this error because it is not 'scp' and in the format it is expecting.
This is why you see this error.