diff --git a/raillabel_providerkit/validation/validate_onthology/_onthology_classes/_attributes/_attribute_abc.py b/raillabel_providerkit/validation/validate_onthology/_onthology_classes/_attributes/_attribute_abc.py
index ae6b9d5..6edebc5 100644
--- a/raillabel_providerkit/validation/validate_onthology/_onthology_classes/_attributes/_attribute_abc.py
+++ b/raillabel_providerkit/validation/validate_onthology/_onthology_classes/_attributes/_attribute_abc.py
@@ -12,11 +12,13 @@
 
 @dataclass
 class _Attribute(abc.ABC):
-    @abc.abstractclassmethod
+    @classmethod
+    @abc.abstractmethod
     def supports(cls, data_dict: dict) -> bool:
         raise NotImplementedError
 
-    @abc.abstractclassmethod
+    @classmethod
+    @abc.abstractmethod
     def fromdict(cls, data_dict: dict) -> t.Type["_Attribute"]:
         raise NotImplementedError
 
diff --git a/raillabel_providerkit/validation/validate_onthology/_onthology_classes/_onthology.py b/raillabel_providerkit/validation/validate_onthology/_onthology_classes/_onthology.py
index 15e319e..456f0b2 100644
--- a/raillabel_providerkit/validation/validate_onthology/_onthology_classes/_onthology.py
+++ b/raillabel_providerkit/validation/validate_onthology/_onthology_classes/_onthology.py
@@ -12,6 +12,7 @@
 @dataclass
 class _Onthology:
     classes: t.Dict[str, _ObjectClass]
+    errors = []
 
     @classmethod
     def fromdict(cls, data_dict: dict) -> "_Onthology":