diff --git a/.github/workflows/tests.yaml b/.github/workflows/tests.yaml
index e5b74d5b49..4111715cf2 100644
--- a/.github/workflows/tests.yaml
+++ b/.github/workflows/tests.yaml
@@ -142,9 +142,9 @@ jobs:
             python continuous_integration/scripts/parse_stdout.py < reports/stdout > reports/pytest.xml
           fi
 
-      # - name: Debug with tmate on failure
-      #   if: ${{ failure() }}
-      #   uses: mxschmitt/action-tmate@v3
+      - name: Debug with tmate on failure
+        if: ${{ failure() }}
+        uses: mxschmitt/action-tmate@v3
 
       - name: Coverage
         uses: codecov/codecov-action@v1
diff --git a/distributed/scheduler.py b/distributed/scheduler.py
index 524a060ea9..a3e243465a 100644
--- a/distributed/scheduler.py
+++ b/distributed/scheduler.py
@@ -2379,6 +2379,7 @@ def _transition(self, key, finish: str, *args, **kwargs):
                 stimulus_id = scheduler.STIMULUS_ID.get()
             except LookupError:
                 if self._validate:
+                    # Can't pickle ContextVars
                     raise LookupError(scheduler.STIMULUS_ID.name)
                 else:
                     stimulus_id = "<stimulus_id unset>"