Garrard GC-910 to Denon DRA-F100 problem

Hooking my Garrard GC-910 turntable to Denon DRA-F100, I get a loud hum (the same whether on a record or not), and then the receiver displays "PROTECT". There's no grounding screw on the turntable. I've tried to play with touching a grounding wire (hooked to the 'signal ground' on the receiver) to various spots on the turntable, but that "PROTECT" mode happens so quick there's no play to experiment.

Any suggestions?