diff --git a/msgpack/_packer.pyx b/msgpack/_packer.pyx
@@ -288,6 +288,10 @@ cdef class Packer:
     @cython.critical_section
     def pack_ext_type(self, typecode, data):
         self._check_exports()
+        if not isinstance(typecode, int):
+            raise TypeError("typecode must have int type.")
+        if not 0 <= typecode <= 127:
+            raise ValueError("typecode should be 0-127")
         if len(data) > ITEM_LIMIT:
             raise ValueError("ext data too large")
         msgpack_pack_ext(&self.pk, typecode, len(data))

diff --git a/test/test_extension.py b/test/test_extension.py
@@ -1,5 +1,7 @@
 import array
 
+import pytest
+
 import msgpack
 from msgpack import ExtType
 
@@ -85,3 +87,40 @@ def default(obj):
     testout = msgpack.packb(obj, default=default)
 
     assert refout == testout
+
+
+@pytest.mark.parametrize("autoreset", [False, True])
+@pytest.mark.parametrize(
+    "typecode, error",
+    [
+        (-129, ValueError),
+        (-128, ValueError),
+        (-1, ValueError),
+        (128, ValueError),
+        (256, ValueError),
+        (2**100, ValueError),
+        (1.5, TypeError),
+        (None, TypeError),
+        ("1", TypeError),
+    ],
+)
+def test_pack_ext_type_invalid_code(typecode, error, autoreset):
+    packer = msgpack.Packer(autoreset=autoreset)
+    packer.pack(42)
+    before = packer.bytes()
+    with pytest.raises(error):
+        packer.pack_ext_type(typecode, b"A")
+    assert packer.bytes() == before
+
+    packed = packer.pack_ext_type(0, b"A")
+    if not autoreset:
+        packed = packer.bytes()
+        assert packed[:1] == msgpack.packb(42)
+        packed = packed[1:]
+    assert msgpack.unpackb(packed) == ExtType(0, b"A")
+
+
+@pytest.mark.parametrize("typecode", [0, 127])
+def test_pack_ext_type_code_boundary(typecode):
+    packed = msgpack.Packer().pack_ext_type(typecode, b"A")
+    assert msgpack.unpackb(packed) == ExtType(typecode, b"A")