$ python3.3
Python 3.3.2+ (default, Feb 28 2014, 00:52:16) 
[GCC 4.8.1] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import numpy
>>> import numpy, numpy.ma, numpy.version
>>> numpy.version.version
'1.8.0'
>>> A = numpy.empty(shape=(5,), dtype=[("A", "<f4"), ("B", "datetime64[ms]")])
>>> A.view(numpy.ma.MaskedArray)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/gerrit/venv/python-3.3/lib/python3.3/site-packages/numpy/ma/core.py", line 2800, in __array_finalize__
    self._fill_value = _check_fill_value(None, self.dtype)
  File "/home/gerrit/venv/python-3.3/lib/python3.3/site-packages/numpy/ma/core.py", line 402, in _check_fill_value
    dtype=ndtype,)
ValueError: Error parsing datetime string "?" at position 0