Use -1 as a sentinel.

stepmania/src/arch/InputHandler/InputHandler_Carbon.cpp

@@ -182,6 +182,7 @@ void InputHandler_Carbon::AddDevices( int usagePage, int usage, InputDevice &id
 	while( (device = IOIteratorNext(iter)) )
 	{
 		HIDDevice *dev = MakeDevice( id );
+		int num;
 		
 		if( !dev )
 		{
@@ -189,14 +190,12 @@ void InputHandler_Carbon::AddDevices( int usagePage, int usage, InputDevice &id
 			continue;
 		}
 		
-		if( !dev->Open(device) )
+		if( !dev->Open(device) || (num = dev->AssignIDs(id)) == -1 )
 		{
 			delete dev;
 			IOObjectRelease( device );
 			continue;
 		}
-		
-		int num = dev->AssignIDs( id );
 		io_iterator_t i;
 		
 		enum_add( id, num );

stepmania/src/archutils/Darwin/HIDDevice.h

@@ -103,7 +103,8 @@ public:
 	/*
 	 * Returns the number of IDs assigned starting from startID. This is not meaningful for devices like
 	 * keyboards that all share the same InputDevice id. If a particular device has multiple logical
-	 * devices, then it must ensure that AssignIDs does not assign an ID outside of its range.
+	 * devices, then it must ensure that AssignIDs does not assign an ID outside of its range. Return
+	 * -1 to indicate that the device does not share the same InputDevice and none could be assigned.
 	 */
 	virtual int AssignIDs( InputDevice startID ) { return 0; }