USB2ANY: When trying to output the serial number of the device, it outputs a negative number

class USB2ANY:
    u2adll = U2A_ERRWrapper(ctypes.WinDLL(_DLL_PATH))
    
    @classmethod
    def _FindControllers(cls) -> int:
        '''
        Scan the USB, and return the total number of USB2ANY found.
        '''
        return cls.u2adll.u2aFindControllers()

    @classmethod
    def _GetSerialNumber(cls, index: int) -> ctypes.c_buffer:
        '''
        Returns a ctypes.c_buffer containing the ascii bytes corresponding to 
        the serial number of a USB2ANY at the given index, with respect to 
        the ordering determined by _FindControllers function.
        '''
        buf = ctypes.c_buffer(40) # `40` is from SERNUM_LEN in USB2ANY_SDK.h
        status = cls.u2adll.u2aGetSerialNumber(index, buf)
        return buf

    @classmethod
    def Controllers(cls) -> List[ctypes.c_buffer]:
        '''
        Returns a list of ctypes.c_buffers containing the serial numbers of 
        all detected USB2ANY.
        '''
        return [cls._GetSerialNumber(i)
                for i in range(cls._FindControllers())]

    def Open(self, controller: Optional[ctypes.c_buffer] = None) -> None:
        '''
        Acquires a handle to the controller specified by the serial number. 
        If controller argument is omitted, the handle will be acquired for 
        the first USB2ANY enumerated.
    
        On failure, the handle is not assigned, and an exception is raised.
        '''
        if controller is None: # use an empty buffer to select first U2A
            controller = ctypes.c_buffer(0)
        self._handle = self.u2adll.u2aOpen(controller)