diff --git a/swh/model/identifiers.py b/swh/model/identifiers.py
index 0254a89ca2a07f760253a2be785e9be6bc900acc..6a00068145034d088cc3240fb20136df7b227a23 100644
--- a/swh/model/identifiers.py
+++ b/swh/model/identifiers.py
@@ -638,14 +638,17 @@ _object_type_map = {
 }
 
 
-class PersistentId(NamedTuple(
-        'PersistentId', [
-            ('namespace', str),
-            ('scheme_version', int),
-            ('object_type', str),
-            ('object_id', str),
-            ('metadata', Dict[str, Any]),
-        ])):
+_PersistentId = NamedTuple(
+    'PersistentId', [
+        ('namespace', str),
+        ('scheme_version', int),
+        ('object_type', str),
+        ('object_id', str),
+        ('metadata', Dict[str, Any]),
+    ])
+
+
+class PersistentId(_PersistentId):
     """
     Named tuple holding the relevant info associated to a Software Heritage
     persistent identifier.
diff --git a/swh/model/merkle.py b/swh/model/merkle.py
index 15c5db62b7bc907efdc4f842db8781c164a2dc0f..02c6f2b29d17e5f6d9dc5336fe760bfc68d1617e 100644
--- a/swh/model/merkle.py
+++ b/swh/model/merkle.py
@@ -110,7 +110,7 @@ class MerkleNode(dict, metaclass=abc.ABCMeta):
     """
     __slots__ = ['parents', 'data', '__hash', 'collected']
 
-    type = None  # type: Optional[str]
+    type = None  # type: Optional[str]  # TODO: make this an enum
     """Type of the current node (used as a classifier for :func:`collect`)"""
 
     def __init__(self, data=None):