diff --git a/src/planai/joined_task.py b/src/planai/joined_task.py
index 5059af6..6ad67d2 100644
--- a/src/planai/joined_task.py
+++ b/src/planai/joined_task.py
@@ -63,7 +63,8 @@ def consume_work(self, task: Task):
         prefix = task.prefix_for_input_task(self.join_type)
         if prefix is None:
             raise ValueError(
-                f"Task {task} does not have a prefix for {self.join_type.__name__} in provenance."
+                f"Task {task} does not have a prefix for {self.join_type.__name__} in provenance. "
+                f"Existing provenance: {task._provenance}"
             )
         need_watch = False
         with self._lock: