diff --git a/python/src/rowing_data/codec.py b/python/src/rowing_data/codec.py
index 47250a4..55407a3 100644
--- a/python/src/rowing_data/codec.py
+++ b/python/src/rowing_data/codec.py
@@ -4,12 +4,14 @@
 ``physical = raw / scale`` and ``raw = round(physical * scale)``.
 
 Absent values are ``None`` and MUST be omitted from the file — never encoded
-as a placeholder such as -1 or 999.
+as a placeholder such as -1 or 999. Each base type's FIT invalid value (spec
+§7.2) means "no data": ``encode`` never produces it for a measurement, and
+``decode`` returns ``None`` for it.
 """
 
 from __future__ import annotations
 
-from .fields import BASE_TYPE_RANGE, FieldDef
+from .fields import BASE_TYPE_INVALID, BASE_TYPE_RANGE, FieldDef
 
 
 class CodecError(ValueError):
@@ -27,12 +29,15 @@ def encode(field: FieldDef, physical: float | int | None) -> int | None:
             f"{field.name}: encoded value {raw} does not fit {field.base_type} "
             f"range [{lo}, {hi}] (physical={physical!r}, scale={field.scale})"
         )
+    invalid = BASE_TYPE_INVALID[field.base_type]
+    if raw == invalid:
+        return invalid - 1
     return raw
 
 
 def decode(field: FieldDef, raw: int | None) -> float | int | None:
     """Convert a FIT raw integer to a physical value, or ``None`` if absent."""
-    if raw is None:
+    if raw is None or raw == BASE_TYPE_INVALID[field.base_type]:
         return None
     if field.scale == 1:
         return int(raw)
diff --git a/python/src/rowing_data/fields.py b/python/src/rowing_data/fields.py
index 313e330..cafe710 100644
--- a/python/src/rowing_data/fields.py
+++ b/python/src/rowing_data/fields.py
@@ -40,6 +40,15 @@ class BaseType(StrEnum):
     BaseType.SINT32: (-2_147_483_648, 2_147_483_647),
 }
 
+# FIT invalid value per base type: it means "no data" and is never a measurement.
+BASE_TYPE_INVALID: dict[BaseType, int] = {
+    BaseType.UINT8: 255,
+    BaseType.UINT16: 65535,
+    BaseType.UINT32: 4_294_967_295,
+    BaseType.SINT16: 32767,
+    BaseType.SINT32: 2_147_483_647,
+}
+
 
 @dataclass(frozen=True, slots=True)
 class FieldDef:
diff --git a/python/tests/test_codec.py b/python/tests/test_codec.py
index adc1533..609b42c 100644
--- a/python/tests/test_codec.py
+++ b/python/tests/test_codec.py
@@ -74,6 +74,22 @@ def test_encode_overflow_raises() -> None:
 
 def test_uint16_upper_bound_accepted() -> None:
     work = field_by_id(19)
-    assert encode(work, 65535) == 65535
+    assert encode(work, 65534) == 65534
     with pytest.raises(CodecError):
         encode(work, 65536)
+
+
+@pytest.mark.parametrize(
+    ("name", "invalid"),
+    [
+        ("StrokeWork", 65535),  # UINT16
+        ("WorkoutState", 255),  # UINT8
+    ],
+)
+def test_invalid_value_is_never_written_as_a_measurement(
+    name: str, invalid: int
+) -> None:
+    field = field_by_name(name)
+    assert encode(field, invalid / field.scale) == invalid - 1
+    assert decode(field, invalid) is None
+    assert decode(field, invalid - 1) is not None
diff --git a/python/tests/test_reader.py b/python/tests/test_reader.py
index 8507ca7..93cdc3e 100644
--- a/python/tests/test_reader.py
+++ b/python/tests/test_reader.py
@@ -240,6 +240,51 @@ def test_unknown_stroke_state_is_unknown(tmp_path: Path) -> None:
     assert any(issue.code == "stroke_state" for issue in loaded.read_issues)
 
 
+def test_invalid_value_is_read_as_missing(tmp_path: Path) -> None:
+    path = tmp_path / "invalid.fit"
+    builder = FitFileBuilder(auto_define=True, min_string_size=64)
+    file_id = FileIdMessage()
+    file_id.type = FileType.ACTIVITY
+    file_id.manufacturer = Manufacturer.DEVELOPMENT
+    file_id.time_created = _ts_ms()
+    builder.add(file_id)
+    dev_id = DeveloperDataIdMessage()
+    dev_id.application_id = APPLICATION_ID
+    dev_id.developer_data_index = 0
+    builder.add(dev_id)
+    desc = FieldDescriptionMessage()
+    desc.developer_data_index = 0
+    desc.field_definition_number = 19
+    desc.fit_base_type_id = BaseType.UINT16.value
+    desc.field_name = "StrokeWork"
+    desc.scale = 1
+    desc.offset = 0
+    desc.units = ""
+    builder.add(desc)
+    work = DeveloperField(
+        developer_data_index=0,
+        field_id=19,
+        size=2,
+        name="StrokeWork",
+        base_type=BaseType.UINT16,
+        scale=1,
+        offset=0,
+        units="",
+    )
+    work.set_value(0, 65535)
+    session = SessionMessage()
+    session.start_time = _ts_ms()
+    session.timestamp = _ts_ms()
+    builder.add(session)
+    rec = RecordMessage(developer_fields=[work])
+    rec.timestamp = _ts_ms()
+    builder.add(rec)
+    builder.build().to_file(str(path))
+
+    loaded = read_fit(path)
+    assert loaded.records[0].stroke_work_j is None
+
+
 def test_invalid_recording_strategy_is_unknown_with_warning(tmp_path: Path) -> None:
     path = tmp_path / "bad_strategy.fit"
     builder = FitFileBuilder(auto_define=True, min_string_size=64)