diff --git a/ably/pubsub/realtime/presencemap.py b/ably/pubsub/realtime/presencemap.py
index a5d2c427..654fa77d 100644
--- a/ably/pubsub/realtime/presencemap.py
+++ b/ably/pubsub/realtime/presencemap.py
@@ -37,8 +37,9 @@ def _is_newer(item: PresenceMessage, existing: PresenceMessage) -> bool:
     Raises:
         ValueError: If message ids cannot be parsed for comparison
     """
-    # RTP2b1: if either is synthesized, compare by timestamp
-    if item.is_synthesized() or existing.is_synthesized():
+    # RTP2b1: if either is synthesized, compare by timestamp. A message without an id
+    # cannot be compared by msgSerial and index, so it takes the same path.
+    if item.is_synthesized() or existing.is_synthesized() or not item.id or not existing.id:
         # RTP2b1a: if equal, prefer the newly-arrived one (item)
         if item.timestamp is None and existing.timestamp is None:
             return True
diff --git a/ably/pubsub/types/message.py b/ably/pubsub/types/message.py
index aaf1b4d6..9cc8fcb6 100644
--- a/ably/pubsub/types/message.py
+++ b/ably/pubsub/types/message.py
@@ -370,7 +370,7 @@ def from_encoded(obj, cipher=None, context=None):
 
     @staticmethod
     def __update_empty_fields(proto_msg: dict, msg: dict, msg_index: int):
-        if msg.get("id") is None or msg.get("id") == '':
+        if (msg.get("id") is None or msg.get("id") == '') and proto_msg.get('id'):
             msg['id'] = f"{proto_msg.get('id')}:{msg_index}"
         if msg.get("connectionId") is None or msg.get("connectionId") == '':
             msg['connectionId'] = proto_msg.get('connectionId')
diff --git a/test/unit/message_test.py b/test/unit/message_test.py
index 22e0b53a..99534713 100644
--- a/test/unit/message_test.py
+++ b/test/unit/message_test.py
@@ -95,3 +95,29 @@ def test_validate_message_size_raises_40009_above_the_limit():
 
     assert excinfo.value.code == 40009
     assert excinfo.value.status_code == 400
+
+
+# TM2a
+def test_update_inner_message_fields_does_not_fabricate_id_without_protocol_id():
+    proto_msg: dict = {
+        'connectionId': 'custom_connection_id',
+        'presence': [{'action': 2, 'clientId': 'c1'}],
+    }
+    ably.pubsub.types.message.Message.update_inner_message_fields(proto_msg)
+    assert proto_msg['presence'][0].get('id') is None
+
+
+# RTP2b
+def test_presence_without_any_id_is_compared_by_timestamp():
+    from ably.pubsub.realtime.presencemap import _is_newer
+    from ably.pubsub.types.presence import PresenceMessage
+
+    def build(timestamp):
+        proto_msg = {'action': 14, 'timestamp': timestamp,
+                     'presence': [{'action': 2, 'clientId': 'c1'}]}
+        ably.pubsub.types.message.Message.update_inner_message_fields(proto_msg)
+        return PresenceMessage.from_encoded(proto_msg['presence'][0])
+
+    older, newer = build(1000), build(2000)
+    assert _is_newer(newer, older) is True
+    assert _is_newer(older, newer) is False