diff --git a/dpgen2/exploration/selector/conf_filter.py b/dpgen2/exploration/selector/conf_filter.py
index 77bac2b2..3fca483f 100644
--- a/dpgen2/exploration/selector/conf_filter.py
+++ b/dpgen2/exploration/selector/conf_filter.py
@@ -72,9 +72,10 @@ def check(
         self,
         ms: dpdata.MultiSystems,
     ) -> dpdata.MultiSystems:
-        selected_idx = sum(
-            [[(i, j) for j in range(s.get_nframes())] for i, s in enumerate(ms)], []
-        )
+        selected_idx = []
+        for i in range(len(ms)):
+            for j in range(ms[i].get_nframes()):
+                selected_idx.append((i, j))
         for ff in self._filters:
             res = ff.batched_check([ms[i][j] for i, j in selected_idx])
             selected_idx = [idx for i, idx in enumerate(selected_idx) if res[i]]