diff -r 96a68e369d13 Doc/library/functions.rst
--- a/Doc/library/functions.rst	Mon Dec 09 20:33:33 2013 -0700
+++ b/Doc/library/functions.rst	Tue Dec 10 12:29:08 2013 +0800
@@ -1178,8 +1178,8 @@
 .. function:: round(number[, ndigits])
 
    Return the floating point value *number* rounded to *ndigits* digits after
-   the decimal point.  If *ndigits* is omitted, it defaults to zero. Delegates
-   to ``number.__round__(ndigits)``.
+   the decimal point.  If *ndigits* is omitted, it returns the nearest integer
+   to its input.  Delegates to ``number.__round__(ndigits)``.
 
    For the built-in types supporting :func:`round`, values are rounded to the
    closest multiple of 10 to the power minus *ndigits*; if two multiples are
diff -r 96a68e369d13 Lib/test/test_float.py
--- a/Lib/test/test_float.py	Mon Dec 09 20:33:33 2013 -0700
+++ b/Lib/test/test_float.py	Tue Dec 10 12:29:08 2013 +0800
@@ -769,6 +769,14 @@
             test(sfmt, NAN, ' nan')
             test(sfmt, -NAN, ' nan')
 
+    def test_None_ndigits(self):
+        for x in round(1.23), round(1.23, None), round(1.23, ndigits=None):
+            self.assertEqual(x, 1)
+            self.assertIsInstance(x, int)
+        for x in round(1.78), round(1.78, None), round(1.78, ndigits=None):
+            self.assertEqual(x, 2)
+            self.assertIsInstance(x, int)
+
 
 # Beginning with Python 2.6 float has cross platform compatible
 # ways to create and represent inf and nan
diff -r 96a68e369d13 Objects/floatobject.c
--- a/Objects/floatobject.c	Mon Dec 09 20:33:33 2013 -0700
+++ b/Objects/floatobject.c	Tue Dec 10 12:29:08 2013 +0800
@@ -979,8 +979,9 @@
     x = PyFloat_AsDouble(v);
     if (!PyArg_ParseTuple(args, "|O", &o_ndigits))
         return NULL;
-    if (o_ndigits == NULL) {
-        /* single-argument round: round to nearest integer */
+    if (o_ndigits == NULL || o_ndigits == Py_None) {
+        /* single-argument round or with None ndigits:
+         * round to nearest integer */
         rounded = round(x);
         if (fabs(x-rounded) == 0.5)
             /* halfway case: round to even */