Undefined variable in metrics.py

parlai/core/metrics.py

@@ -1046,7 +1046,7 @@ def _consume_user_metrics(self, observation):
                 if uk in ALL_METRICS:
                     # don't let the user override our metrics
                     uk = f'USER_{uk}'
-                assert isinstance(uk, str), type(k)
+                assert isinstance(uk, str), f'{type(uk)} is not a str'
                 if not isinstance(v, Metric):
                     warn_once(f'Metric {uk} is assumed to be averaged per example.')
                     v = AverageMetric(v)