def METHOD_NAME(self):
    """Test that filtering a subject's metrics by a non-existing tag returns None."""
    metric = Metric(self.DATA_MODEL, {"tags": ["tag"]}, METRIC_ID)
    subject = Subject(
        self.DATA_MODEL,
        {"name": "Subject", "metrics": {METRIC_ID: metric}},
        SUBJECT_ID,
        self.report,
    )
    self.assertIsNone(subject.tag_subject("unused"))