diff --git a/msgpack/_packer.pyx b/msgpack/_packer.pyx
@@ -291,6 +291,10 @@ cdef class Packer:
            raise ValueError("ext data too large")
        msgpack_pack_ext(&self.pk, typecode, len(data))
        msgpack_pack_raw_body(&self.pk, data, len(data))

    @cython.critical_section
    def pack_array_header(self, long long size):

diff --git a/msgpack/fallback.py b/msgpack/fallback.py
@@ -861,6 +861,10 @@ def pack_ext_type(self, typecode, data):
            self._buffer.write(b"\xc9" + struct.pack(">I", L))
        self._buffer.write(struct.pack("B", typecode))
        self._buffer.write(data)

    def _pack_array_header(self, n):
        if n <= 0x0F:

diff --git a/pyproject.toml b/pyproject.toml
@@ -19,6 +19,7 @@ classifiers = [
    "Programming Language :: Python :: Implementation :: CPython",
    "Programming Language :: Python :: Implementation :: PyPy",
]

[project.urls]
Homepage = "https://msgpack.org/"
@@ -43,3 +44,9 @@ lint.select = [
    "I", # isort
    #"UP", pyupgrade
]