diff --git a/Stackless/changelog.txt b/Stackless/changelog.txt
index a1ca85d6f373ac..8dd92930d4dd91 100644
--- a/Stackless/changelog.txt
+++ b/Stackless/changelog.txt
@@ -9,6 +9,10 @@ What's New in Stackless 3.X.X?
 
 *Release date: 20XX-XX-XX*
 
+- https://github.com/stackless-dev/stackless/issues/265
+  Avoid undefined behavior, if Stackless classes implement __reduce__() and
+  __reduce_ex__() using a single C-function.
+
 - https://github.com/stackless-dev/stackless/issues/265
   Fix C-API functions PyChannel_Send(), PyChannel_Send_nr(),
   PyChannel_Receive(), PyChannel_Receive_nr(), and
diff --git a/Stackless/core/cframeobject.c b/Stackless/core/cframeobject.c
index fe2ce011a39831..dc1a8500e85a8b 100644
--- a/Stackless/core/cframeobject.c
+++ b/Stackless/core/cframeobject.c
@@ -133,7 +133,7 @@ static PyObject * execute_soft_switchable_func(PyFrameObject *, int, PyObject *)
 SLP_DEF_INVALID_EXEC(execute_soft_switchable_func)
 
 static PyObject *
-cframe_reduce(PyCFrameObject *cf)
+cframe_reduce(PyCFrameObject *cf, PyObject *value)
 {
     PyObject *res = NULL, *exec_name = NULL;
     PyObject *params = NULL;
@@ -141,6 +141,11 @@ cframe_reduce(PyCFrameObject *cf)
     PyObject *obs[3];
     long i, n;
 
+    if (value && !PyLong_Check(value)) {
+        PyErr_SetString(PyExc_TypeError, "__reduce_ex__ argument should be an integer");
+        return NULL;
+    }
+
     if (cf->f_execute == execute_soft_switchable_func) {
         exec_name = (PyObject *) cf->any2;
         assert(cf->any2);
@@ -240,7 +245,7 @@ cframe_setstate(PyObject *self, PyObject *args)
 
 static PyMethodDef cframe_methods[] = {
     {"__reduce__",    (PyCFunction)cframe_reduce, METH_NOARGS, NULL},
-    {"__reduce_ex__", (PyCFunction)cframe_reduce, METH_VARARGS, NULL},
+    {"__reduce_ex__", (PyCFunction)cframe_reduce, METH_O, NULL},
     {"__setstate__",  (PyCFunction)cframe_setstate, METH_O, NULL},
     {NULL, NULL}
 };
@@ -386,8 +391,13 @@ slp_cframe_fini(void)
  */
 
 static PyObject *
-function_declaration_reduce(PyStacklessFunctionDeclarationObject *self)
+function_declaration_reduce(PyStacklessFunctionDeclarationObject *self, PyObject *value)
 {
+    if (value && !PyLong_Check(value)) {
+        PyErr_SetString(PyExc_TypeError, "__reduce_ex__ argument should be an integer");
+        return NULL;
+    }
+
     if (self->name == NULL || *self->name == '\0') {
         PyErr_SetString(PyExc_SystemError, "no function name");
         return NULL;
@@ -397,7 +407,7 @@ function_declaration_reduce(PyStacklessFunctionDeclarationObject *self)
 
 static PyMethodDef function_declaration_methods[] = {
     {"__reduce__",    (PyCFunction)function_declaration_reduce, METH_NOARGS, NULL},
-    {"__reduce_ex__", (PyCFunction)function_declaration_reduce, METH_VARARGS, NULL},
+    {"__reduce_ex__", (PyCFunction)function_declaration_reduce, METH_O, NULL},
     {NULL, NULL}
 };
 
diff --git a/Stackless/module/channelobject.c b/Stackless/module/channelobject.c
index b7f8173813373f..1d524323b4da33 100644
--- a/Stackless/module/channelobject.c
+++ b/Stackless/module/channelobject.c
@@ -1104,12 +1104,17 @@ PyDoc_STRVAR(channel_reduce__doc__,
 "channel.__reduce__() -- currently does not distinguish threads.");
 
 static PyObject *
-channel_reduce(PyChannelObject * ch)
+channel_reduce(PyChannelObject * ch, PyObject *value)
 {
     PyObject *tup = NULL, *lis = NULL;
     PyTaskletObject *t;
     int i, n;
 
+    if (value && !PyLong_Check(value)) {
+        PyErr_SetString(PyExc_TypeError, "__reduce_ex__ argument should be an integer");
+        return NULL;
+    }
+
     lis = PyList_New(0);
     if (lis == NULL) goto err_exit;
     t = ch->head;
@@ -1188,7 +1193,7 @@ channel_methods[] = {
     channel_open__doc__},
     {"__reduce__",              (PCF)channel_reduce,        METH_NOARGS,
      channel_reduce__doc__},
-    {"__reduce_ex__",           (PCF)channel_reduce,        METH_VARARGS,
+    {"__reduce_ex__",           (PCF)channel_reduce,        METH_O,
      channel_reduce__doc__},
     {"__setstate__",            (PCF)channel_setstate,      METH_O,
      channel_setstate__doc__},
diff --git a/Stackless/module/scheduling.c b/Stackless/module/scheduling.c
index 4f8a39d4957e14..e10dfef32d07ed 100644
--- a/Stackless/module/scheduling.c
+++ b/Stackless/module/scheduling.c
@@ -216,10 +216,15 @@ slp_bomb_explode(PyObject *_bomb)
 }
 
 static PyObject *
-bomb_reduce(PyBombObject *bomb)
+bomb_reduce(PyBombObject *bomb, PyObject *value)
 {
     PyObject *tup;
 
+    if (value && !PyLong_Check(value)) {
+        PyErr_SetString(PyExc_TypeError, "__reduce_ex__ argument should be an integer");
+        return NULL;
+    }
+
     tup = slp_into_tuple_with_nulls(&bomb->curexc_type, 3);
     if (tup != NULL)
         tup = Py_BuildValue("(O()O)", &PyBomb_Type, tup);
@@ -246,7 +251,7 @@ static PyMemberDef bomb_members[] = {
 
 static PyMethodDef bomb_methods[] = {
     {"__reduce__",              (PyCFunction)bomb_reduce,       METH_NOARGS},
-    {"__reduce_ex__",           (PyCFunction)bomb_reduce,       METH_VARARGS},
+    {"__reduce_ex__",           (PyCFunction)bomb_reduce,       METH_O},
     {"__setstate__",            (PyCFunction)bomb_setstate,     METH_O},
     {NULL,     NULL}             /* sentinel */
 };
diff --git a/Stackless/module/stacklessmodule.c b/Stackless/module/stacklessmodule.c
index f723ea0b44a17c..a6a4472bd083e1 100644
--- a/Stackless/module/stacklessmodule.c
+++ b/Stackless/module/stacklessmodule.c
@@ -855,8 +855,13 @@ get_thread_info(PyObject *self, PyObject *args)
 }
 
 static PyObject *
-slpmodule_reduce(PyObject *self)
+slpmodule_reduce(PyObject *self, PyObject *value)
 {
+    if (value && !PyLong_Check(value)) {
+        PyErr_SetString(PyExc_TypeError, "__reduce_ex__ argument should be an integer");
+        return NULL;
+    }
+
     return PyUnicode_FromString("_stackless");
 }
 
@@ -1709,7 +1714,7 @@ static PyMethodDef stackless_methods[] = {
      _get_all_objects__doc__},
 #endif
     {"__reduce__",                  (PCF)slpmodule_reduce,      METH_NOARGS, NULL},
-    {"__reduce_ex__",               (PCF)slpmodule_reduce,      METH_VARARGS, NULL},
+    {"__reduce_ex__",               (PCF)slpmodule_reduce,      METH_O, NULL},
     {"getdebug",                   (PCF)slpmodule_getdebug,    METH_NOARGS,
     slpmodule_getdebug__doc__},
     {"getuncollectables",          (PCF)slpmodule_getuncollectables,    METH_NOARGS,
diff --git a/Stackless/module/taskletobject.c b/Stackless/module/taskletobject.c
index 958f4d4d98accc..5ee791d76c7c15 100644
--- a/Stackless/module/taskletobject.c
+++ b/Stackless/module/taskletobject.c
@@ -612,7 +612,7 @@ simply the tasklet() call without parameters.
 */
 
 static PyObject *
-tasklet_reduce(PyTaskletObject * t)
+tasklet_reduce(PyTaskletObject * t, PyObject *value)
 {
     PyObject *tup = NULL, *lis = NULL;
     PyFrameObject *f;
@@ -620,6 +620,11 @@ tasklet_reduce(PyTaskletObject * t)
     PyObject *exc_type, *exc_value, *exc_traceback, *exc_info;
     PyObject *context = NULL;
 
+    if (value && !PyLong_Check(value)) {
+        PyErr_SetString(PyExc_TypeError, "__reduce_ex__ argument should be an integer");
+        return NULL;
+    }
+
     if (ts && t == ts->st.current)
         RUNTIME_ERROR("You cannot __reduce__ the tasklet which is"
                       " current.", NULL);
@@ -2580,7 +2585,7 @@ static PyMethodDef tasklet_methods[] = {
      tasklet_setup__doc__},
     {"__reduce__",              (PCF)tasklet_reduce,        METH_NOARGS,
      tasklet_reduce__doc__},
-    {"__reduce_ex__",           (PCF)tasklet_reduce,        METH_VARARGS,
+    {"__reduce_ex__",           (PCF)tasklet_reduce,        METH_O,
      tasklet_reduce__doc__},
     {"__setstate__",            (PCF)tasklet_setstate,      METH_O,
      tasklet_setstate__doc__},