diff --git a/src/geouned/GEOUNED/utils/boolean_function.py b/src/geouned/GEOUNED/utils/boolean_function.py
index a9bc2e4a..1bfde6a1 100644
--- a/src/geouned/GEOUNED/utils/boolean_function.py
+++ b/src/geouned/GEOUNED/utils/boolean_function.py
@@ -624,7 +624,7 @@ def group_single(self):
     def get_surfaces_numbers(self):
         """Return the list of all surfaces in the BoolSequence definition."""
         if type(self.elements) is bool:
-            return tuple()
+            return set()
         surf = set()
         for e in self.elements:
             if type(e) is int: