Be more assertive about missing arguments to unsubscribe

Change-Id: Ic77263b8a98a5d129b7ae8566dc7d8a42e120295
Overlooked a nonexistent variable reference

Followup to Iafc6fcaec8aadeb81edc1fd3a4b815f4f2acb232

Change-Id: I2157369cbbb5fa80614c900082eed9a0c779730a