diff --git a/yt/utilities/hierarchy_inspection.py b/yt/utilities/hierarchy_inspection.py
index 7e62ed1684f..8a2e0aae92f 100644
--- a/yt/utilities/hierarchy_inspection.py
+++ b/yt/utilities/hierarchy_inspection.py
@@ -39,4 +39,4 @@ def find_lowest_subclasses(candidates: List[Type]) -> List[Type]:
 
     count = reduce(lambda x, y: x + y, counters)
 
-    return [x for x in count.keys() if count[x] == 1]
+    return [x for x in candidates if count[x] == 1]
diff --git a/yt/utilities/tests/test_hierarchy_inspection.py b/yt/utilities/tests/test_hierarchy_inspection.py
index 8f7d7510e61..c5d36f90952 100644
--- a/yt/utilities/tests/test_hierarchy_inspection.py
+++ b/yt/utilities/tests/test_hierarchy_inspection.py
@@ -55,3 +55,9 @@ def test_diverging_tree():
     result = find_lowest_subclasses([level1, level2, level3, level1a])
     assert len(result) == 2
     assert level1a in result and level3 in result
+
+
+def test_without_parents():
+    result = find_lowest_subclasses([level1, level3])
+    assert len(result) == 1
+    assert level3 in result