diff --git a/dpgen2/exploration/task/caly_task_group.py b/dpgen2/exploration/task/caly_task_group.py
index 14594f38..a903f502 100644
--- a/dpgen2/exploration/task/caly_task_group.py
+++ b/dpgen2/exploration/task/caly_task_group.py
@@ -122,7 +122,7 @@ def set_params(
             for temp in name_of_atoms[1:]:
                 overlap = overlap & set(temp)
 
-            if any(map(lambda s: (set(s) - overlap) == 0, name_of_atoms)):
+            if any(not (set(atom_choices) - overlap) for atom_choices in name_of_atoms):
                 raise ValueError(
                     f"Any sub-list should not equal with intersection, e.g. [[A,B,C], [B,C], [C]] is not allowed."
                 )
diff --git a/tests/exploration/test_make_task_group_from_config.py b/tests/exploration/test_make_task_group_from_config.py
index f9fe93be..0fc33b30 100644
--- a/tests/exploration/test_make_task_group_from_config.py
+++ b/tests/exploration/test_make_task_group_from_config.py
@@ -126,3 +126,15 @@ def test_make_caly_input(self):
     def test_caly_task_group(self):
         tgroup = make_calypso_task_group_from_config(self.config)
         self.assertTrue(isinstance(tgroup, CalyTaskGroup))
+
+    def test_rejects_impossible_random_atom_choices(self):
+        """Fail before random selection when unique choices are impossible."""
+        config = {
+            "name_of_atoms": [["Li"], ["Li"]],
+            "numb_of_atoms": [10, 10],
+            "numb_of_species": 2,
+            "distance_of_ions": [[1.0, 1.0], [1.0, 1.0]],
+        }
+
+        with self.assertRaisesRegex(ValueError, "intersection"):
+            make_calypso_task_group_from_config(config)