diff --git a/rdagent/scenarios/data_science/loop.py b/rdagent/scenarios/data_science/loop.py
index dffe173c..4738996f 100644
--- a/rdagent/scenarios/data_science/loop.py
+++ b/rdagent/scenarios/data_science/loop.py
@@ -311,7 +311,7 @@ class DataScienceRDLoop(RDLoop):
 
             # only clean current workspace without affecting other loops.
             for k in "direct_exp_gen", "coding", "running":
-                if k in prev_out:
+                if k in prev_out and prev_out[k] is not None:
                     assert isinstance(prev_out[k], DSExperiment)
                     clean_workspace(prev_out[k].experiment_workspace.workspace_path)