def _ensure_required_inputs(self):
        """
        Raises ValueError if required inputs are not present
        """
        for name, input_type in self._input_types.items():
            if not input_type.optional and self._input_vars[name] is None:
                msg_prefix = 'Op "{}" (op_type: {}) '.format(self.name, self.op_type)
                raise ValueError(
                    msg_prefix + "Required input {} is missing".format(name)
                )

        Raises ValueError if required inputs are not present
        
coremltools/converters/mil/mil/operation.py