diff --git a/aleph_message/models/__init__.py b/aleph_message/models/__init__.py
index 6570a0d..5e31440 100644
--- a/aleph_message/models/__init__.py
+++ b/aleph_message/models/__init__.py
@@ -70,7 +70,7 @@ class ChainRef(BaseModel):
     sender: str
     signature: str
     time: float
-    type = "POST"
+    type: Literal["POST"] = "POST"
 
 
 class MessageConfirmationHash(BaseModel):