VPI layer can return misleading information about cocotb HOT 1 CLOSED

ERROR: called callback function returned NULL
Traceback (most recent call last):
  File "/home/saw/work/cocotb/cocotb/scheduler.py", line 87, in react
    self.schedule(coroutine, trigger=trigger)
  File "/home/saw/work/cocotb/cocotb/scheduler.py", line 203, in schedule
    result = coroutine.send(trigger)
  File "/home/saw/work/cocotb/cocotb/decorators.py", line 128, in send
    return self._coro.send(value)
  File "/home/saw/work/modena_new/cognento/library/AOEpps/testbench/sf_streaming.py", line 116, in _monitor_recv
    if self.bus.valid.value and self.bus.ready.value:
  File "/home/saw/work/cocotb/cocotb/handle.py", line 67, in getvalue
    result.binstr = self._get_value_str()
  File "/home/saw/work/cocotb/cocotb/binary.py", line 120, in set_binstr
    raise ValueError("Attempting to assign character %s to a %s (full value: %s)" % (char, self.__class__.__name__, string))
ValueError: Attempting to assign character N to a BinaryValue (full value: UNKNOWN)
Failed to execute callback
           V C S   S i m u l a t i o n   R e p o r t
Time: 2500
CPU Time:      0.270 seconds;       Data structure size:   0.0Mb