diff --git a/scim2_models/base.py b/scim2_models/base.py
index f3a6781..6890547 100644
--- a/scim2_models/base.py
+++ b/scim2_models/base.py
@@ -706,6 +706,8 @@ class ComplexAttribute(BaseModel):
     """A complex attribute as defined in :rfc:`RFC7643 §2.3.8
     <7643#section-2.3.8>`."""
 
+    _schema: Optional[str] = None
+
     def get_attribute_urn(self, field_name: str) -> str:
         """Build the full URN of the attribute.