diff --git a/CHANGELOG.md b/CHANGELOG.md
index 0dde66bee..26edf36b1 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3,6 +3,7 @@ All notable changes to this project will be documented in this file.
 This project uses the changelog in accordance with [keepchangelog](http://keepachangelog.com/). Please use this to write notable changes, which is not the same as git commit log...
 
 ## [unreleased][unreleased]
+- Fixed `tools/ePassport` - a document number longer than nine characters left its overflow in the MRZ optional-data field, where it was reported as the holder's personal number (@pkilar)
 - Added `tools/ePassport` - EF_DG13 is decoded, and a Polish document's PESEL is shown as the personal number when it carries no EF_DG11 (@pkilar)
 - Fixed `tools/ePassport` - a disabled button drew Kivy's banded disabled texture, which left its label unreadable (@pkilar)
 - Fixed `hf 14b` - a card asking for a waiting time extension of 4 or more left the timeout at zero, so any ISO14443-B card needing time to answer looked like it had stopped responding (@pkilar)
diff --git a/tools/ePassport/epassport/emrtd/mrz.py b/tools/ePassport/epassport/emrtd/mrz.py
index 85a5cbf05..d84420d70 100644
--- a/tools/ePassport/epassport/emrtd/mrz.py
+++ b/tools/ePassport/epassport/emrtd/mrz.py
@@ -483,8 +483,12 @@ def _parse_td3(lines: list[str], today: _dt.date | None) -> Mrz:
         date_of_birth=Checked(l2[13:19], l2[19], verify(l2[13:19], l2[19])),
         sex=l2[20],
         date_of_expiry=Checked(l2[21:27], l2[27], verify(l2[21:27], l2[27])),
+        # The value comes from what _extended_document_number left behind: a
+        # long document number spills into this field, and that spill belongs
+        # to the number, not to the State's optional data.  The check digit
+        # still answers for the field as transmitted, overflow included.
         optional_data=Checked(
-            l2[28:42].rstrip(FILLER), l2[42], verify(l2[28:42], l2[42])
+            optional.rstrip(FILLER), l2[42], verify(l2[28:42], l2[42])
         ),
         composite=Checked(
             "",
diff --git a/tools/ePassport/tests/test_mrz.py b/tools/ePassport/tests/test_mrz.py
index 29d26f1b1..4725812c3 100644
--- a/tools/ePassport/tests/test_mrz.py
+++ b/tools/ePassport/tests/test_mrz.py
@@ -193,3 +193,67 @@ def test_split_names_handles_single_and_missing_given_names() -> None:
     assert mrz.split_names("ERIKSSON<<ANNA<MARIA<<<<") == ("ERIKSSON", "ANNA MARIA")
     assert mrz.split_names("ERIKSSON<<<<<<") == ("ERIKSSON", "")
     assert mrz.split_names("VAN<DER<BERG<<JAN<<<") == ("VAN DER BERG", "JAN")
+
+
+# --------------------------------- document numbers longer than nine characters
+def _td3_with_long_number(number: str = "AB1234567890") -> list[str]:
+    """ICAO puts the overflow in the optional-data field.
+
+    Positions 1-9 hold the first nine characters, position 10 a filler in
+    place of the check digit, and the optional-data field opens with the rest
+    of the number followed by the check digit for the whole of it.
+    """
+    head, rest = number[:9], number[9:]
+    dob, expiry = "740812", "120415"
+    optional = (rest + mrz.check_digit(number)).ljust(14, "<")
+    body = (
+        head
+        + "<"
+        + "UTO"
+        + dob
+        + mrz.check_digit(dob)
+        + "F"
+        + expiry
+        + mrz.check_digit(expiry)
+        + optional
+        + mrz.check_digit(optional)
+    )
+    line2 = body + mrz.check_digit(body[0:10] + body[13:20] + body[21:43])
+    return ["P<UTOERIKSSON<<ANNA<MARIA".ljust(44, "<"), line2]
+
+
+def test_a_long_document_number_is_put_back_together() -> None:
+    parsed = mrz.parse(_td3_with_long_number())
+    assert parsed.document_number.value == "AB1234567890"
+    assert parsed.document_number.ok is True
+
+
+def test_the_overflow_does_not_linger_in_the_optional_field() -> None:
+    """It is part of the document number, not data the State chose to add.
+
+    Left there it was reported as the holder's personal number, since that
+    falls back to the MRZ optional-data field when DG11 carries none.
+    """
+    parsed = mrz.parse(_td3_with_long_number())
+    assert parsed.optional_data.value == ""
+
+
+def test_a_nine_character_number_leaves_the_optional_field_alone() -> None:
+    dob, expiry = "740812", "120415"
+    optional = "ZE184226B".ljust(14, "<")
+    body = (
+        "L898902C"
+        + "<"
+        + mrz.check_digit("L898902C<")
+        + "UTO"
+        + dob
+        + mrz.check_digit(dob)
+        + "F"
+        + expiry
+        + mrz.check_digit(expiry)
+        + optional
+        + mrz.check_digit(optional)
+    )
+    line2 = body + mrz.check_digit(body[0:10] + body[13:20] + body[21:43])
+    parsed = mrz.parse(["P<UTOERIKSSON<<ANNA".ljust(44, "<"), line2])
+    assert parsed.optional_data.value == "ZE184226B"