diff --git a/src/cnlpt/cnlp_data.py b/src/cnlpt/cnlp_data.py
index 64cead10..6216278f 100644
--- a/src/cnlpt/cnlp_data.py
+++ b/src/cnlpt/cnlp_data.py
@@ -742,7 +742,7 @@ def _reconcile_labels_lists(self, processor):
                     raise Exception("The same task name has disjoint sets of labels in different dataset: %s vs. %s" % (str(old_labels), str(new_labels)))
                 elif new_labels != old_labels:
                     logger.warn("Two different datasets have the same task name but not completely equal label lists: %s vs. %s. We will merge them." % (str(old_labels), str(new_labels)))
-                    self.tasks_to_labels[task] = old_labels.union(new_labels)
+                    self.tasks_to_labels[task] = list(old_labels.union(new_labels))
                 else:
                     ## they match completely, nothing to be done
                     pass