diff --git a/pcapkit/corekit/fields/strings.py b/pcapkit/corekit/fields/strings.py
index 3760293d64..d3199cfe57 100644
--- a/pcapkit/corekit/fields/strings.py
+++ b/pcapkit/corekit/fields/strings.py
@@ -8,6 +8,7 @@
 
 from pcapkit.corekit.fields.field import Field, NoValue
 from pcapkit.utilities.compat import Dict
+from pcapkit.utilities.exceptions import FieldValueError
 
 __all__ = [
     '_TextField',
@@ -196,6 +197,19 @@ def __init__(self, length: 'int',
 
         self._namespace = namespace or {}
 
+        # NOTE: A subfield reaching past the end of the field is a mistake in the
+        # schema that declares it, not something a packet can cause, so it is
+        # rejected here rather than at packing time -- where it used to grow the
+        # buffer instead, and surface much later as an opaque ``OverflowError``
+        # from :meth:`int.to_bytes`.
+        width = self.length * 8
+        for name, (start, size) in self._namespace.items():
+            if start < 0 or size < 1 or start + size > width:
+                raise FieldValueError(
+                    f'{type(self).__name__}: subfield {name!r} spans bits '
+                    f'{start}-{start + size - 1} of a {width}-bit field'
+                )
+
     def pre_process(self, value: 'dict[str, Any]', packet: 'dict[str, Any]') -> 'bytes':  # pylint: disable=unused-argument
         """Process field value before construction (packing).
 
@@ -206,12 +220,26 @@ def pre_process(self, value: 'dict[str, Any]', packet: 'dict[str, Any]') -> 'byt
         Returns:
             Processed field value.
 
+        Raises:
+            FieldValueError: If a subfield value does not fit the bits it was
+                declared to occupy.
+
         """
-        buffer = bytearray(self.length * 8)
-        for name, (start, len) in self._namespace.items():
-            end = start + len
-            buffer[start:end] = f'{value[name]:0{end - start}b}'.encode()
-        return int(b''.join(map(lambda x: b'1' if x else b'0', buffer)), 2).to_bytes(self.length, 'big')
+        # NOTE: The buffer holds one ASCII digit per bit, so it must be seeded with
+        # ``b'0'`` rather than with NUL bytes -- a zero bit written by the loop below
+        # is the *character* ``b'0'``, which is itself non-zero, so a truthiness test
+        # over the buffer cannot tell a cleared bit from a set one and would report
+        # every named bit as set.
+        buffer = bytearray(b'0' * (self.length * 8))
+        for name, (start, size) in self._namespace.items():
+            bits = f'{value[name]:0{size}b}'
+            if len(bits) > size:
+                raise FieldValueError(
+                    f'{type(self).__name__}: subfield {name!r} value {value[name]!r} '
+                    f'needs {len(bits)} bits, but was declared with {size}'
+                )
+            buffer[start:start + size] = bits.encode()
+        return int(buffer, 2).to_bytes(self.length, 'big')
 
     def post_process(self, value: 'bytes', packet: 'dict[str, Any]') -> 'dict[str, Any]':  # pylint: disable=unused-argument
         """Process field value after parsing (unpacked).
diff --git a/pcapkit/protocols/schema/internet/ipv4.py b/pcapkit/protocols/schema/internet/ipv4.py
index 02b891159c..3afcbed688 100644
--- a/pcapkit/protocols/schema/internet/ipv4.py
+++ b/pcapkit/protocols/schema/internet/ipv4.py
@@ -526,7 +526,7 @@ class IPv4(Schema):
     #: Version and header length.
     vihl: 'VerIHLField' = BitField(length=1, namespace={
         'version': (0, 4),
-        'ihl': (4, 8),
+        'ihl': (4, 4),
     })
     #: Type of service.
     tos: 'ToSField' = BitField(length=1, namespace={
diff --git a/tests/corekit/test_fields_strings.py b/tests/corekit/test_fields_strings.py
new file mode 100644
index 0000000000..a5e4b07eee
--- /dev/null
+++ b/tests/corekit/test_fields_strings.py
@@ -0,0 +1,104 @@
+from __future__ import annotations
+
+import unittest
+
+from tests._support import purge_modules
+
+
+class BitFieldTests(unittest.TestCase):
+    """Packing and parsing of :class:`~pcapkit.corekit.fields.strings.BitField`."""
+
+    def setUp(self) -> None:
+        purge_modules(['pcapkit'])
+
+        from pcapkit.corekit.fields.strings import BitField
+
+        self.BitField = BitField
+        #: One octet of single-bit flags, as the protocol schemas declare them.
+        self.flags = BitField(length=1, namespace={
+            'a': (0, 1),
+            'b': (1, 1),
+            'c': (6, 1),
+            'd': (7, 1),
+        })
+
+    def test_cleared_bits_stay_cleared_when_a_sibling_is_set(self) -> None:
+        """A zero bit must not be packed as a one.
+
+        The buffer holds one ASCII digit per bit, and ``b'0'`` is itself a
+        non-zero byte, so a truthiness test over it reported every named bit as
+        set regardless of its value.
+
+        """
+        self.assertEqual(self.flags.pre_process({'a': 0, 'b': 0, 'c': 0, 'd': 1}, {}), b'\x01')
+        self.assertEqual(self.flags.pre_process({'a': 1, 'b': 0, 'c': 0, 'd': 0}, {}), b'\x80')
+        self.assertEqual(self.flags.pre_process({'a': 0, 'b': 1, 'c': 1, 'd': 0}, {}), b'\x42')
+
+    def test_no_bits_set_packs_to_zero(self) -> None:
+        self.assertEqual(self.flags.pre_process({'a': 0, 'b': 0, 'c': 0, 'd': 0}, {}), b'\x00')
+
+    def test_every_bit_set_packs_to_its_mask(self) -> None:
+        self.assertEqual(self.flags.pre_process({'a': 1, 'b': 1, 'c': 1, 'd': 1}, {}), b'\xc3')
+
+    def test_multi_bit_subfields_keep_their_own_width(self) -> None:
+        field = self.BitField(length=1, namespace={'kind': (0, 3), 'value': (3, 5)})
+
+        self.assertEqual(field.pre_process({'kind': 5, 'value': 17}, {}), b'\xb1')
+        self.assertEqual(field.pre_process({'kind': 0, 'value': 1}, {}), b'\x01')
+
+    def test_unnamed_bits_are_left_alone(self) -> None:
+        """Bits outside the namespace belong to nobody and must pack as zero."""
+        field = self.BitField(length=2, namespace={'flag': (3, 1)})
+
+        self.assertEqual(field.pre_process({'flag': 1}, {}), b'\x10\x00')
+        self.assertEqual(field.pre_process({'flag': 0}, {}), b'\x00\x00')
+
+    def test_a_subfield_reaching_past_the_field_is_rejected_at_declaration(self) -> None:
+        """A namespace entry wider than the field is a schema mistake.
+
+        Slice assignment on a :class:`bytearray` *grows* it, so declaring
+        ``ihl: (4, 8)`` on a one-octet field built a 12-bit buffer and then
+        failed in :meth:`int.to_bytes` with an opaque ``OverflowError``, far
+        from the declaration at fault. ``pcapkit/protocols/schema/internet/
+        ipv4.py`` really did carry that declaration.
+
+        """
+        from pcapkit.utilities.exceptions import FieldValueError
+
+        with self.assertRaises(FieldValueError):
+            self.BitField(length=1, namespace={'version': (0, 4), 'ihl': (4, 8)})
+        with self.assertRaises(FieldValueError):
+            self.BitField(length=1, namespace={'flag': (8, 1)})
+        with self.assertRaises(FieldValueError):
+            self.BitField(length=1, namespace={'flag': (-1, 1)})
+
+    def test_a_value_too_wide_for_its_subfield_is_rejected(self) -> None:
+        from pcapkit.utilities.exceptions import FieldValueError
+
+        field = self.BitField(length=1, namespace={'kind': (0, 3), 'value': (3, 5)})
+
+        with self.assertRaises(FieldValueError):
+            field.pre_process({'kind': 8, 'value': 0}, {})
+        self.assertEqual(field.pre_process({'kind': 7, 'value': 31}, {}), b'\xff')
+
+    def test_ipv4_version_and_ihl_pack_into_one_octet(self) -> None:
+        """The real declaration this guard was written for."""
+        from pcapkit.protocols.schema.internet.ipv4 import IPv4
+
+        field = IPv4.__fields__['vihl']
+
+        self.assertEqual(field.pre_process({'version': 4, 'ihl': 5}, {}), b'\x45')
+        self.assertEqual(field.post_process(b'\x45', {}), {'version': 4, 'ihl': 5})
+
+    def test_pack_and_parse_round_trip(self) -> None:
+        for value in ({'a': 0, 'b': 0, 'c': 0, 'd': 0},
+                      {'a': 0, 'b': 0, 'c': 0, 'd': 1},
+                      {'a': 1, 'b': 0, 'c': 1, 'd': 0},
+                      {'a': 1, 'b': 1, 'c': 1, 'd': 1}):
+            with self.subTest(value=value):
+                packed = self.flags.pre_process(value, {})
+                self.assertEqual(self.flags.post_process(packed, {}), value)
+
+
+if __name__ == '__main__':
+    unittest.main()