diff --git a/sisyphus/job.py b/sisyphus/job.py
index de4ca58..7f901ae 100644
--- a/sisyphus/job.py
+++ b/sisyphus/job.py
@@ -121,7 +121,11 @@ def __call__(cls, *args, **kwargs):
                 job._sis_add_block(b)
 
         # Update alias prefixes
-        job._sis_alias_prefixes.add(gs.ALIAS_AND_OUTPUT_SUBDIR)
+        try:
+            job._sis_alias_prefixes.add(gs.ALIAS_AND_OUTPUT_SUBDIR)
+        except Exception as e:
+            assert False, ("An empty Job was loaded, this can happen if classes were passed as parameters that could"
+                           "not be pickled properly")
 
         # add stacktrace information, if set to None or -1 use full stack
         stack_depth = gs.JOB_ADD_STACKTRACE_WITH_DEPTH + 1 if gs.JOB_ADD_STACKTRACE_WITH_DEPTH >= 0 else None