diff --git a/src/edr_pydantic_classes/observed_property.py b/src/edr_pydantic_classes/observed_property.py
index e515507..f6ea60c 100644
--- a/src/edr_pydantic_classes/observed_property.py
+++ b/src/edr_pydantic_classes/observed_property.py
@@ -11,7 +11,7 @@ class Category(EDRBaseModel):
 
 
 class ObservedProperty(EDRBaseModel):
-    id: str = None
+    id: Optional[str] = None
     label: str
     description: Optional[str] = None
     categories: Optional[List[Category]] = None