diff --git a/modules/deeplearning/icing.py b/modules/deeplearning/icing.py
index 7c5f387297783d5af1fe7c558e6e9f205d63b5a4..7b1a8dec874338525583d97f6bd0e7673ce68671 100644
--- a/modules/deeplearning/icing.py
+++ b/modules/deeplearning/icing.py
@@ -179,7 +179,7 @@ class IcingIntensityNN:
                 print(e)
 
     def get_in_mem_data_batch(self, idxs):
-        key = set(idxs)
+        key = frozenset(idxs)
 
         if CACHE_DATA_IN_MEM:
             data, label = self.in_mem_data_cache.get(key)