Fail on invalid channel group.
diff --git a/device.c b/device.c
index 90c9fc3..55d306b 100644
--- a/device.c
+++ b/device.c
@@ -133,6 +133,7 @@
 			return cg;
 		}
 	}
+	g_critical("Invalid channel group '%s'", opt_channel_group);
 
 	return NULL;
 }