NumPy recombination join_by TypeError

I am facing a TypeError when I try to join a 'uint16' field to a structured array in NumPy 1.11 or 1.12 (Python 3.5).

import numpy as np
from numpy.lib import recfunctions as rfn
foo = np.array([(1,)],
               dtype=[('key', int)])
bar = np.array([(1,np.array([1,2,3]))],
               dtype=[('key', int), ('value', 'uint16', 3)])
rfn.join_by('key', foo, bar)

      

        
        
        
      

    

This is mistake:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/user/anaconda3/lib/python3.5/site-packages/numpy/lib/recfunctions.py", line 986, in join_by
    output.sort(order=key)
  File "/home/user/anaconda3/lib/python3.5/site-packages/numpy/ma/core.py", line 5420, in sort
    sidx = self.filled(filler).argsort(axis=axis, kind=kind,
  File "/home/user/anaconda3/lib/python3.5/site-packages/numpy/ma/core.py", line 3668, in filled
    fill_value = _check_fill_value(fill_value, self.dtype)
  File "/home/user/anaconda3/lib/python3.5/site-packages/numpy/ma/core.py", line 470, in _check_fill_value
    fill_value = np.array(_recursive_set_fill_value(fill_value, ndtype),
  File "/home/user/anaconda3/lib/python3.5/site-packages/numpy/ma/core.py", line 436, in _recursive_set_fill_value
    output_value.append(np.array(fval, dtype=cdtype).item())
TypeError: int() argument must be a string, a bytes-like object or a number, not 'NoneType'

      

        
        
        
      

    

The same problem does not occur if I use 'float16'.

import numpy as np
from numpy.lib import recfunctions as rfn
foo = np.array([(1,)],
               dtype=[('key', int)])
bar = np.array([(1,np.array([1,2,3]))],
               dtype=[('key', int), ('value', 'float16', 3)])
rfn.join_by('key', foo, bar)

      

        
        
        
      

    

Is this just a mistake? Or is there a way to prevent this problem?