diff -r 40e8b39199da -r 12b5ea4f89bc Python/getargs.c
--- a/Python/getargs.c	Sat Dec 10 17:34:46 2016 +0100
+++ b/Python/getargs.c	Sat Dec 10 18:15:18 2016 +0100
@@ -741,20 +741,23 @@
     }
 
     case 'n': /* Py_ssize_t */
+    case 'N': /* Optional Py_ssize_t */
     {
         PyObject *iobj;
         Py_ssize_t *p = va_arg(*p_va, Py_ssize_t *);
         Py_ssize_t ival = -1;
-        if (float_argument_error(arg))
-            RETURN_ERR_OCCURRED;
-        iobj = PyNumber_Index(arg);
-        if (iobj != NULL) {
-            ival = PyLong_AsSsize_t(iobj);
-            Py_DECREF(iobj);
+        if (c == 'n' || arg != Py_None) {
+            if (float_argument_error(arg))
+                RETURN_ERR_OCCURRED;
+            iobj = PyNumber_Index(arg);
+            if (iobj != NULL) {
+                ival = PyLong_AsSsize_t(iobj);
+                Py_DECREF(iobj);
+            }
+            if (ival == -1 && PyErr_Occurred())
+                RETURN_ERR_OCCURRED;
+            *p = ival;
         }
-        if (ival == -1 && PyErr_Occurred())
-            RETURN_ERR_OCCURRED;
-        *p = ival;
         break;
     }
     case 'l': {/* long int */
@@ -2237,6 +2240,7 @@
     case 'L': /* long long */
     case 'K': /* long long sized bitfield */
     case 'n': /* Py_ssize_t */
+    case 'N': /* Optional Py_ssize_t */
     case 'f': /* float */
     case 'd': /* double */
     case 'D': /* complex double */
diff -r 40e8b39199da -r 12b5ea4f89bc Tools/clinic/clinic.py
--- a/Tools/clinic/clinic.py	Sat Dec 10 17:34:46 2016 +0100
+++ b/Tools/clinic/clinic.py	Sat Dec 10 18:15:18 2016 +0100
@@ -2617,10 +2617,13 @@
 
 class Py_ssize_t_converter(CConverter):
     type = 'Py_ssize_t'
-    default_type = int
+    default_type = (int, NoneType)
     format_unit = 'n'
     c_ignored_default = "0"
 
+    def converter_init(self):
+        if self.default is None:
+            self.format_unit = 'N'
 
 class float_converter(CConverter):
     type = 'float'