diff -r 4e67dce9721b Objects/methodobject.c
--- a/Objects/methodobject.c	Mon Jan 16 23:52:00 2017 +0100
+++ b/Objects/methodobject.c	Tue Jan 17 09:06:02 2017 +0900
@@ -237,15 +237,21 @@
     case METH_FASTCALL:
     {
         PyObject **stack;
         PyObject *kwnames;
         _PyCFunctionFast fastmeth = (_PyCFunctionFast)meth;
 
-        stack = _PyStack_UnpackDict(args, nargs, kwargs, &kwnames, func_obj);
-        if (stack == NULL) {
-            return NULL;
+        if (nargs) {
+            stack = _PyStack_UnpackDict(args, nargs, kwargs, &kwnames, func_obj);
+            if (stack == NULL) {
+                return NULL;
+            }
+        }
+        else {
+            stack = args;
+            kwnames = NULL;
         }
 
         result = (*fastmeth) (self, stack, nargs, kwnames);
         if (stack != args) {
             PyMem_Free(stack);
         }
diff -r 4e67dce9721b Tools/clinic/clinic.py
--- a/Tools/clinic/clinic.py	Mon Jan 16 23:52:00 2017 +0100
+++ b/Tools/clinic/clinic.py	Tue Jan 17 09:06:02 2017 +0900
@@ -814,64 +814,59 @@
             # positional parameters with option groups
             # (we have to generate lots of PyArg_ParseTuple calls
             #  in a big switch statement)
 
             flags = "METH_VARARGS"
             parser_prototype = parser_prototype_varargs
-
             parser_definition = parser_body(parser_prototype, '    {option_group_parsing}')
 
         elif positional and all_boring_objects:
             # positional-only, but no option groups,
             # and nothing but normal objects:
             # PyArg_UnpackTuple!
-
             flags = "METH_VARARGS"
             parser_prototype = parser_prototype_varargs
 
             parser_definition = parser_body(parser_prototype, normalize_snippet("""
                 if (!PyArg_UnpackTuple(args, "{name}",
                     {unpack_min}, {unpack_max},
                     {parse_arguments})) {{
                     goto exit;
                 }}
                 """, indent=4))
 
-        elif positional:
-            # positional-only, but no option groups
-            # we only need one call to PyArg_ParseTuple
-
-            flags = "METH_VARARGS"
-            parser_prototype = parser_prototype_varargs
-
-            parser_definition = parser_body(parser_prototype, normalize_snippet("""
-                if (!PyArg_ParseTuple(args, "{format_units}:{name}",
-                    {parse_arguments})) {{
-                    goto exit;
-                }}
-                """, indent=4))
+            # TODO: Support PyArg_UnpackStack added by http://bugs.python.org/issue29259
 
         elif not new_or_init:
             flags = "METH_FASTCALL"
-
             parser_prototype = parser_prototype_fastcall
-
             body = normalize_snippet("""
                 if (!_PyArg_ParseStack(args, nargs, kwnames, &_parser,
                     {parse_arguments})) {{
                     goto exit;
                 }}
                 """, indent=4)
             parser_definition = parser_body(parser_prototype, body)
             parser_definition = insert_keywords(parser_definition)
+
+        elif positional:
+            # positional-only, but no option groups
+            # we only need one call to PyArg_ParseTuple
+            flags = "METH_VARARGS"
+            parser_prototype = parser_prototype_varargs
+            parser_definition = parser_body(parser_prototype, normalize_snippet("""
+                if (!PyArg_ParseTuple(args, "{format_units}:{name}",
+                    {parse_arguments})) {{
+                    goto exit;
+                }}
+                """, indent=4))
+
         else:
             # positional-or-keyword arguments
             flags = "METH_VARARGS|METH_KEYWORDS"
-
             parser_prototype = parser_prototype_keyword
-
             body = normalize_snippet("""
                 if (!_PyArg_ParseTupleAndKeywordsFast(args, kwargs, &_parser,
                     {parse_arguments})) {{
                     goto exit;
                 }}
                 """, indent=4)