diff --git a/Include/dictobject.h b/Include/dictobject.h
index 08a9449..28930f4 100644
--- a/Include/dictobject.h
+++ b/Include/dictobject.h
@@ -115,7 +115,7 @@
 PyAPI_FUNC(void) _PyDict_MaybeUntrack(PyObject *mp);
 PyAPI_FUNC(int) _PyDict_HasOnlyStringKeys(PyObject *mp);
 Py_ssize_t _PyDict_KeysSize(PyDictKeysObject *keys);
-Py_ssize_t _PyDict_SizeOf(PyDictObject *);
+PyAPI_FUNC(Py_ssize_t) _PyDict_SizeOf(PyDictObject *);
 PyAPI_FUNC(PyObject *) _PyDict_Pop(PyObject *, PyObject *, PyObject *);
 PyObject *_PyDict_Pop_KnownHash(PyObject *, PyObject *, Py_hash_t, PyObject *);
 PyObject *_PyDict_FromKeys(PyObject *, PyObject *, PyObject *);