GX420t - SGD command not executed



I'm using a GX420t printer connected via TCP/IP. The firmware is V56.17.17Z. I'm trying to figure out whether an optional cutter is installed with sending the SGD command

! U1 getvar "device.cutter_installed"

Instead of answering with "Yes" or "No" the printer responds with "?". What's going wrong?

Thanks for any hint.