diff --git a/python/packages/core/agent_framework/_threads.py b/python/packages/core/agent_framework/_threads.py
index f7603a7c3c..92469a78d5 100644
--- a/python/packages/core/agent_framework/_threads.py
+++ b/python/packages/core/agent_framework/_threads.py
@@ -140,6 +140,7 @@ def __init__(
         """
         if not messages:
             self.messages: list[ChatMessage] = []
+            return
         if not isinstance(messages, list):
             raise TypeError("Messages should be a list")
         new_messages: list[ChatMessage] = []
diff --git a/python/packages/core/tests/core/test_threads.py b/python/packages/core/tests/core/test_threads.py
index 8049501789..492ed11519 100644
--- a/python/packages/core/tests/core/test_threads.py
+++ b/python/packages/core/tests/core/test_threads.py
@@ -384,6 +384,18 @@ def test_init_empty(self) -> None:
 
         assert len(state.messages) == 0
 
+    def test_init_none(self) -> None:
+        """Test ChatMessageStoreState initialization with None messages."""
+        state = ChatMessageStoreState(messages=None)
+
+        assert len(state.messages) == 0
+
+    def test_init_no_messages_arg(self) -> None:
+        """Test ChatMessageStoreState initialization without messages argument."""
+        state = ChatMessageStoreState()
+
+        assert len(state.messages) == 0
+
 
 class TestThreadState:
     """Test cases for AgentThreadState class."""
@@ -415,3 +427,22 @@ def test_init_defaults(self) -> None:
 
         assert state.service_thread_id is None
         assert state.chat_message_store_state is None
+
+    def test_init_with_chat_message_store_state_no_messages(self) -> None:
+        """Test AgentThreadState initialization with chat_message_store_state without messages field.
+
+        This tests the scenario where a custom ChatMessageStore (like RedisChatMessageStore)
+        serializes its state without a 'messages' field, containing only configuration data
+        like thread_id, redis_url, etc.
+        """
+        store_data: dict[str, Any] = {
+            "type": "redis_store_state",
+            "thread_id": "test_thread_123",
+            "redis_url": "redis://localhost:6379",
+            "key_prefix": "chat_messages",
+        }
+        state = AgentThreadState.from_dict({"chat_message_store_state": store_data})
+
+        assert state.service_thread_id is None
+        assert state.chat_message_store_state is not None
+        assert state.chat_message_store_state.messages == []
diff --git a/python/packages/redis/tests/test_redis_chat_message_store.py b/python/packages/redis/tests/test_redis_chat_message_store.py
index e445b09809..fa403eb2fe 100644
--- a/python/packages/redis/tests/test_redis_chat_message_store.py
+++ b/python/packages/redis/tests/test_redis_chat_message_store.py
@@ -495,3 +495,20 @@ async def test_extend(self, redis_store, mock_redis_client, sample_messages):
         # Verify rpush was called for each message
         pipeline_mock = mock_redis_client.pipeline.return_value.__aenter__.return_value
         assert pipeline_mock.rpush.call_count >= 2
+
+    async def test_serialize_with_agent_thread(self, redis_store, sample_messages):
+        """Test that RedisChatMessageStore can be serialized within an AgentThread.
+
+        This test verifies the fix for issue #1991 where calling thread.serialize()
+        with a RedisChatMessageStore would fail with "Messages should be a list" error.
+        """
+        from agent_framework import AgentThread
+
+        thread = AgentThread(message_store=redis_store)
+        await thread.on_new_messages(sample_messages)
+
+        serialized = await thread.serialize()
+
+        assert serialized is not None
+        assert "chat_message_store_state" in serialized
+        assert serialized["chat_message_store_state"] is not None