diff --git a/src/ndi/element_timeseries.py b/src/ndi/element_timeseries.py
index 7ec5bec1..56ad7b1e 100644
--- a/src/ndi/element_timeseries.py
+++ b/src/ndi/element_timeseries.py
@@ -287,10 +287,28 @@ def addepoch(
         ``ndi.element.timeseries/addepoch``; ``add_to_database`` is the
         explicit form of that, since Python has no ``nargout``.
         """
-        has_data = timepoints is not None and datapoints is not None
-
-        if not has_data or self._session is None:
-            return super().addepoch(epoch_id, epoch_clock, t0_t1, add_to_database=add_to_database)
+        # THE BINARY IS ALWAYS WRITTEN AND ALWAYS ATTACHED, empty or not.
+        #
+        # This used to skip both when timepoints or datapoints was None and
+        # fall through to the base class, which added an element_epoch with
+        # nothing bound to epoch_binary_data.vhsb. MATLAB has no such branch:
+        # in ndi.element.timeseries/addepoch the vhsb_write and the add_file
+        # sit after the argument handling rather than inside it, and
+        # timepoints/datapoints are required positional arguments, so the
+        # class cannot produce a fileless epoch document at all.
+        #
+        # An epoch with NO SAMPLES is a supported case -- see
+        # ndi/+mock/+fun/subject_stimulator_neuron.m, which calls addepoch
+        # with [] for both -- but its binary is still written, just empty.
+        # An epoch may have no data; it may not have no file. That is exactly
+        # what element_epoch's schema says with mustbenotempty on the file,
+        # and DID enforces it now that DID-python#86 closed check_files'
+        # step-3 fail-open. Before that the invalid document was accepted at
+        # add time and only noticed on read.
+        if timepoints is None:
+            timepoints = np.zeros((0, 1))
+        if datapoints is None:
+            datapoints = np.zeros((0, 1))
 
         _, doc = super().addepoch(epoch_id, epoch_clock, t0_t1, add_to_database=False)
         doc = self._attach_timeseries_data(doc, timepoints, datapoints)
diff --git a/tests/test_element_timeseries_vhsb.py b/tests/test_element_timeseries_vhsb.py
index a3e312fd..f472f2b6 100644
--- a/tests/test_element_timeseries_vhsb.py
+++ b/tests/test_element_timeseries_vhsb.py
@@ -224,14 +224,41 @@ def test_mismatched_lengths_raise(self, element):
 
 
 class TestEpochWithoutData:
-    def test_addepoch_without_samples_stores_no_binary(self, element):
+    """An epoch may have no DATA. It may not have no FILE.
+
+    These two used to assert the opposite -- that an epoch added without
+    samples stored no binary at all -- which is a document MATLAB cannot
+    produce. ndi.element.timeseries/addepoch writes the vhsb and attaches
+    it after the argument handling rather than inside it, and takes
+    timepoints/datapoints as required positional arguments; an epoch with
+    no samples goes through it as [] and [], and still gets a file, just
+    an empty one (ndi/+mock/+fun/subject_stimulator_neuron.m does exactly
+    that for a stimulator element).
+
+    element_epoch's schema has always said so -- mustbenotempty on
+    epoch_binary_data.vhsb -- and DID simply was not checking until
+    DID-python#86 closed check_files' step-3 fail-open. So the old
+    assertions were pinning a Python-only divergence that produced
+    documents the database now rejects at add time and used to accept and
+    fail on at read time.
+    """
+
+    def test_addepoch_without_samples_still_binds_the_binary(self, element):
         session, elem = element
         _, doc = elem.addepoch("epoch_1", _clock(), [(0.0, 1.0)])
         exists, _ = session.database_existbinarydoc(doc, BINARY_FILE_NAME)
-        assert not exists
+        assert exists, "an epoch with no samples still stores an empty vhsb"
+
+    def test_the_bound_binary_is_declared_on_the_document(self, element):
+        """Bound, not merely written: what DID rejects is a name in
+        file_list with no matching entry in files.file_info."""
+        session, elem = element
+        _, doc = elem.addepoch("epoch_1", _clock(), [(0.0, 1.0)])
+        assert BINARY_FILE_NAME in doc.current_file_list()
 
-    def test_reading_an_epoch_with_no_binary_is_not_an_error(self, element):
-        """It falls through to the underlying element, which is empty here."""
+    def test_reading_an_epoch_with_no_samples_gives_nothing(self, element):
+        """The file is there and empty, so the read is empty rather than
+        an error -- which is what the old test was really about."""
         session, elem = element
         elem, _ = elem.addepoch("epoch_1", _clock(), [(0.0, 1.0)])
         data, times, _ = elem.readtimeseries("epoch_1", -np.inf, np.inf)