diff --git a/Lib/test/test_builtin.py b/Lib/test/test_builtin.py
--- a/Lib/test/test_builtin.py
+++ b/Lib/test/test_builtin.py
@@ -3,6 +3,8 @@
 import ast
 import builtins
 import collections
+import decimal
+import fractions
 import io
 import locale
 import os
@@ -1244,6 +1246,14 @@
         self.assertEqual(round(5e15+2), 5e15+2)
         self.assertEqual(round(5e15+3), 5e15+3)
 
+    def test_bug_27936(self):
+        # Verify that ndigits=None means the same as passing in no argument
+        for x in [1234,
+                  1234.56,
+                  decimal.Decimal('1234.56'),
+                  fractions.Fraction(123456, 100)]:
+            self.assertEqual(round(x, None), round(x))
+
     def test_setattr(self):
         setattr(sys, 'spam', 1)
         self.assertEqual(sys.spam, 1)
diff --git a/Python/bltinmodule.c b/Python/bltinmodule.c
--- a/Python/bltinmodule.c
+++ b/Python/bltinmodule.c
@@ -2043,7 +2043,7 @@
         return NULL;
     }
 
-    if (ndigits == NULL)
+    if (ndigits == NULL || ndigits == Py_None)
         result = PyObject_CallFunctionObjArgs(round, NULL);
     else
         result = PyObject_CallFunctionObjArgs(round, ndigits, NULL);