<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _work_request(self, worker, md5=None):
 """Wrapper for a work_request to workbench"""


        # I'm sure there's a better way to do this
        if not md5 and not self.session.md5:
            return 'Must call worker with an md5 argument...'
        elif not md5:
            md5 = self.session.md5

        # Is the md5 a sample_set?
        if self.workbench.is_sample_set(md5):
            return self.workbench.set_work_request(worker, md5)

        # Make the work_request with worker and md5 args
        try:
            return self.workbench.work_request(worker, md5)
        except zerorpc.exceptions.RemoteError as e:
            return repr_to_str_decorator.r_to_s(self._data_not_found)(e)