diff --git a/data/data_loader.py b/data/data_loader.py
index 12706e8..3d18049 100644
--- a/data/data_loader.py
+++ b/data/data_loader.py
@@ -124,7 +124,7 @@ class DataLoaderMultiAspect():
                 continue
             runts = bucket[-runt_count:]
             del bucket[-runt_count:]
-            matching_default_bucket_key = [DEFAULT_BATCH_ID, key[1], key[2]]
+            matching_default_bucket_key = (DEFAULT_BATCH_ID, key[1], key[2])
             buckets[matching_default_bucket_key].extend(runts)
 
         # handle remaining runts by randomly duplicating items