diff --git a/abl/utils/utils.py b/abl/utils/utils.py
index 65c85fc..05b97a4 100644
--- a/abl/utils/utils.py
+++ b/abl/utils/utils.py
@@ -4,7 +4,10 @@ from collections import OrderedDict
 
 # for multiple predictions, modify from `learn_add.py`
 def flatten(l):
-    return [item for sublist in l for item in flatten(sublist)] if isinstance(l, list) else [l]
+    # return [item for sublist in l for item in flatten(sublist)] if isinstance(l, (list, tuple)) else [l]
+    if not isinstance(l[0], (list, tuple)):
+        return l
+    return [item for sublist in l for item in sublist] if isinstance(l, (list, tuple)) else [l]
     
 # for multiple predictions, modify from `learn_add.py`
 def reform_idx(flatten_pred_res, save_pred_res):