diff --git a/livekit-rtc/livekit/rtc/data_stream.py b/livekit-rtc/livekit/rtc/data_stream.py
index 35bdc6c3..52102127 100644
--- a/livekit-rtc/livekit/rtc/data_stream.py
+++ b/livekit-rtc/livekit/rtc/data_stream.py
@@ -389,7 +389,7 @@ def __init__(
         self,
         local_participant: LocalParticipant,
         topic: str = "",
-        attributes: Optional[Dict[str, str]] = {},
+        attributes: Optional[Dict[str, str]] = None,
         stream_id: str | None = None,
         total_size: int | None = None,
         mime_type: str = "",
@@ -456,16 +456,17 @@ async def _wait_for_callback(
     async def aclose(
         self, *, reason: str = "", attributes: Optional[Dict[str, str]] = None
     ) -> None:
-        if self._closed:
-            raise RuntimeError("Stream already closed")
-        if self._writer_handle is None:
-            raise RuntimeError("Stream is not open")
-        self._closed = True
-        # unregister before sending: the request consumes the handle natively,
-        # so it must not be dropped again by the disconnect cleanup even if the
-        # close itself reports an error
-        self._unregister_open()
-        await self._send_close(reason=reason, attributes=attributes)
+        async with self._write_lock:
+            if self._closed:
+                raise RuntimeError("Stream already closed")
+            if self._writer_handle is None:
+                raise RuntimeError("Stream is not open")
+            self._closed = True
+            # unregister before sending: the request consumes the handle natively,
+            # so it must not be dropped again by the disconnect cleanup even if the
+            # close itself reports an error
+            self._unregister_open()
+            await self._send_close(reason=reason, attributes=attributes)
 
     async def _send_close(self, *, reason: str, attributes: Optional[Dict[str, str]]) -> None:
         raise NotImplementedError
@@ -477,7 +478,7 @@ def __init__(
         local_participant: LocalParticipant,
         *,
         topic: str = "",
-        attributes: Optional[Dict[str, str]] = {},
+        attributes: Optional[Dict[str, str]] = None,
         stream_id: str | None = None,
         total_size: int | None = None,
         reply_to_id: str | None = None,
diff --git a/livekit-rtc/tests/test_data_stream_writer.py b/livekit-rtc/tests/test_data_stream_writer.py
new file mode 100644
index 00000000..7f2bf3f9
--- /dev/null
+++ b/livekit-rtc/tests/test_data_stream_writer.py
@@ -0,0 +1,84 @@
+# Copyright 2026 LiveKit, Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from __future__ import annotations
+
+import asyncio
+from unittest.mock import MagicMock
+
+import pytest
+
+from livekit.rtc.data_stream import TextStreamWriter
+from livekit.rtc.participant import LocalParticipant
+
+
+def test_stream_writer_attributes_default_is_none() -> None:
+    participant = MagicMock(spec=LocalParticipant)
+    participant.identity = "test-user"
+    participant._open_stream_writers = {}
+
+    writer1 = TextStreamWriter(participant, topic="t1")
+    writer2 = TextStreamWriter(participant, topic="t2")
+
+    # attributes default should be None rather than a shared mutable dict
+    assert writer1._options.attributes == {}
+    assert writer2._options.attributes == {}
+
+
+async def test_aclose_waits_for_inflight_write() -> None:
+    participant = MagicMock(spec=LocalParticipant)
+    participant.identity = "test-user"
+    participant._open_stream_writers = {}
+
+    writer = TextStreamWriter(participant, topic="t")
+    writer._register_open(9999)
+
+    order: list[str] = []
+    write_in_progress = asyncio.Event()
+    allow_write_finish = asyncio.Event()
+
+    async def mock_wait_for_callback(req: object, callback_field: str, response_field: str) -> None:
+        if callback_field == "text_stream_writer_write":
+            order.append("write_start")
+            write_in_progress.set()
+            await allow_write_finish.wait()
+            order.append("write_done")
+        elif callback_field == "text_stream_writer_close":
+            order.append("close_sent")
+
+    writer._wait_for_callback = mock_wait_for_callback  # type: ignore[assignment]
+
+    # Start write in background
+    write_task = asyncio.create_task(writer.write("chunk"))
+    await write_in_progress.wait()
+
+    # Initiate aclose while write is still in flight
+    close_task = asyncio.create_task(writer.aclose())
+
+    # Give close_task a chance to run; it must not execute close yet because write holds lock
+    await asyncio.sleep(0.01)
+    assert order == ["write_start"]
+
+    # Now allow write to complete
+    allow_write_finish.set()
+    await write_task
+    await close_task
+
+    # Close must execute only AFTER write completes
+    assert order == ["write_start", "write_done", "close_sent"]
+    assert writer._closed is True
+
+    # Subsequent write must immediately reject
+    with pytest.raises(RuntimeError, match="Cannot write after stream is closed"):
+        await writer.write("late_chunk")