diff --git a/msgpack/_packer.pyx b/msgpack/_packer.pyx
index dcee213..e275ef2 100644
--- a/msgpack/_packer.pyx
+++ b/msgpack/_packer.pyx
@@ -3,6 +3,8 @@
 from cpython cimport *
 from cpython.bytearray cimport PyByteArray_Check, PyByteArray_CheckExact
 
+cdef ExtType
+
 from . import ExtType