diff --git a/sisyphus/job.py b/sisyphus/job.py
index 4d72921..a1db331 100644
--- a/sisyphus/job.py
+++ b/sisyphus/job.py
@@ -109,7 +109,6 @@ def __call__(cls, *args, **kwargs):
         else:
             # create new object
             job = super(Job, cls).__new__(cls)
-            assert isinstance(job, Job)
             job._sis_tags = tags
 
             # store _sis_id