diff --git a/docs/source/pcapkit/foundation/traceflow/tcp.rst b/docs/source/pcapkit/foundation/traceflow/tcp.rst
index 70ccaca55d..dba740148d 100644
--- a/docs/source/pcapkit/foundation/traceflow/tcp.rst
+++ b/docs/source/pcapkit/foundation/traceflow/tcp.rst
@@ -120,7 +120,8 @@ Terminology
 
           (tuple) index
            |--> (Info) data
-           |     |--> 'fpout' : (Optional[str]) output filename if exists
+           |     |--> 'fpout' : (Optional[str]) output filename, or None when
+           |     |                              the format writes no file
            |     |--> 'index': (tuple) tuple of frame index, both directions,
            |     |                     in capture order
            |     |              |--> (int) frame index
diff --git a/pcapkit/foundation/traceflow/data/tcp.py b/pcapkit/foundation/traceflow/data/tcp.py
index 6c34815a5d..a04afd52ba 100644
--- a/pcapkit/foundation/traceflow/data/tcp.py
+++ b/pcapkit/foundation/traceflow/data/tcp.py
@@ -167,7 +167,8 @@ class Index(DeferredPacket, Info):
     #: :class:`~pcapkit.corekit.packet.DeferredPacket` to resolve.
     __deferred__ = Deferred
 
-    #: Output filename if exists.
+    #: Path of the flow's output file, or :data:`None` when the output format
+    #: writes no file. Open and finalised flows report the same path.
     fpout: 'Optional[str]'
     #: Tuple of frame index, **both directions**, in capture order.
     index: 'tuple[int, ...]'
diff --git a/pcapkit/foundation/traceflow/tcp.py b/pcapkit/foundation/traceflow/tcp.py
index 6a2553fafb..9bf1a280a4 100644
--- a/pcapkit/foundation/traceflow/tcp.py
+++ b/pcapkit/foundation/traceflow/tcp.py
@@ -436,6 +436,25 @@ def _make_segment(self, packet: 'Packet[_AT]') -> 'Any':
             timestamp=packet.timestamp,          # capture timestamp
         )
 
+    def _output_path(self, label: 'str') -> 'Optional[str]':
+        """Path of a flow's output file, as an index entry reports it.
+
+        Arguments:
+            label: the flow's label
+
+        Returns:
+            The file :meth:`trace` writes the flow to, or :data:`None` when the
+            output format writes no file at all. An *empty* extension still
+            writes one, named after the label alone.
+
+        Both :meth:`submit` and :meth:`_finalise` report through this, so an
+        open flow and a finalised one name the same file (:issue:`2237`).
+
+        """
+        if self._fdpext is None:
+            return None
+        return f'{self._fproot}/{label}{self._fdpext}'
+
     @staticmethod
     def _ended(buffer: 'Buffer[_AT]') -> 'bool':
         """Whether a flow's connection has been seen to end.
@@ -478,7 +497,7 @@ def _finalise(self, bufid: 'BufferID') -> 'Index':
         logger.debug('TCP flow %s finalised after %d frame(s) (%d forward, %d reverse)',
                      label, len(buf.index), len(buf.forward), len(buf.reverse))
         index = Index(
-            fpout=f'{self._fproot}/{label}{self._fdpext}' if self._fdpext is not None else None,
+            fpout=self._output_path(label),
             index=tuple(buf.index),
             label=label,
             forward=tuple(buf.forward),
@@ -550,7 +569,7 @@ def submit(self) -> 'tuple[Index, ...]':
 
         ret = []  # type: list[Index]
         for buf in self._buffer.values():
-            ret.append(Index(fpout=f"{self._fproot}/{buf.label}{self._fdpext}" if self._fdpext else None,
+            ret.append(Index(fpout=self._output_path(buf.label),
                              index=tuple(buf.index),
                              label=buf.label,
                              forward=tuple(buf.forward),
diff --git a/tests/foundation/traceflow/test_tcp.py b/tests/foundation/traceflow/test_tcp.py
index 5d897d993a..634f3e4c2b 100644
--- a/tests/foundation/traceflow/test_tcp.py
+++ b/tests/foundation/traceflow/test_tcp.py
@@ -552,6 +552,35 @@ def test_tcp_trace_ipv6_label_and_no_extension_output_path(self) -> None:
             self.assertEqual(final_index.index, (1, 2))
             self.assertFalse(pathlib.Path(tempdir, f'{label}.None').exists())
 
+    def test_an_empty_extension_reports_the_written_file_open_or_finalised(self) -> None:
+        """An empty extension still writes a file, and every report names it (#2237).
+
+        The flow file is named after the label alone. :meth:`TCP.submit` reported
+        :data:`None` for a flow still open while :meth:`TCP._finalise` reported the
+        path, so one flow's ``fpout`` depended on when it was read.
+
+        """
+        from dictdumper import JSON
+
+        from pcapkit.foundation.traceflow.tcp import TCP
+
+        TCP.register_dumper('unit-noext', JSON, '')
+
+        with tempfile.TemporaryDirectory() as tempdir:
+            trace = TCP(tempdir, 'unit-noext')
+            label = trace.trace(self._packet(index=1, syn=True))
+            expected = f'{tempdir}/{label}'
+            self.assertTrue(pathlib.Path(expected).is_file())
+
+            # mid-capture: the flow is open, reported by ``submit``
+            open_index, = trace.index
+            self.assertEqual(open_index.fpout, expected)
+
+            # end of capture: the same flow, reported by ``_finalise``
+            trace.finish()
+            final_index, = trace.index
+            self.assertEqual(final_index.fpout, expected)
+
 
 if __name__ == '__main__':
     unittest.main()