diff --git a/openquake/baselib/parallel.py b/openquake/baselib/parallel.py
index <HASH>..<HASH> 100644
--- a/openquake/baselib/parallel.py
+++ b/openquake/baselib/parallel.py
@@ -533,7 +533,7 @@ class Starmap(object):
     calc_id = None
     hdf5 = None
     pids = ()
-    task_ids = ()
+    task_ids = []
 
     @classmethod
     def init(cls, poolsize=None, distribute=OQ_DISTRIBUTE):
Merged from master [skip CI]