diff --git a/Include/cpython/objimpl.h b/Include/cpython/objimpl.h
index 8e3c964..2f802e9 100644
--- a/Include/cpython/objimpl.h
+++ b/Include/cpython/objimpl.h
@@ -138,8 +138,7 @@
 /* Test if a type supports weak references */
 #define PyType_SUPPORTS_WEAKREFS(t) ((t)->tp_weaklistoffset > 0)
 
-#define PyObject_GET_WEAKREFS_LISTPTR(o) \
-    ((PyObject **) (((char *) (o)) + Py_TYPE(o)->tp_weaklistoffset))
+PyAPI_FUNC(PyObject **) PyObject_GET_WEAKREFS_LISTPTR(PyObject *op);
 
 #ifdef __cplusplus
 }