diff --git a/src/pynxtools/dataconverter/validation.py b/src/pynxtools/dataconverter/validation.py
index 2f90b7a19..40f0f9560 100644
--- a/src/pynxtools/dataconverter/validation.py
+++ b/src/pynxtools/dataconverter/validation.py
@@ -631,7 +631,7 @@ def check_attributes_of_nonexisting_field() -> list:
             )
 
     keys_to_remove = check_attributes_of_nonexisting_field()
-    return [not collector.has_validation_problems(), keys_to_remove]
+    return (not collector.has_validation_problems(), keys_to_remove)
 
 
 def populate_full_tree(node: NexusNode, max_depth: Optional[int] = 5, depth: int = 0):