validate the passed dtype

def _validate_dtype(self, dtype):
        """ validate the passed dtype """

        if dtype is not None:
            dtype = pandas_dtype(dtype)

            # a compound dtype
            if dtype.kind == 'V':
                raise NotImplementedError("compound dtypes are not implemented"
                                          " in the {0} constructor"
                                          .format(self.__class__.__name__))

        return dtype