diff -r a4726b5d0365 Lib/inspect.py
--- a/Lib/inspect.py	Sat Mar 01 10:34:22 2014 -0500
+++ b/Lib/inspect.py	Sat Mar 01 11:38:14 2014 -0500
@@ -1125,7 +1125,7 @@
     elif missing == 2:
         s = "{} and {}".format(*names)
     else:
-        tail = ", {} and {}".format(names[-2:])
+        tail = ", {} and {}".format(*names[-2:])
         del names[-2:]
         s = ", ".join(names) + tail
     raise TypeError("%s() missing %i required %s argument%s: %s" %
diff -r a4726b5d0365 Lib/test/test_inspect.py
--- a/Lib/test/test_inspect.py	Sat Mar 01 10:34:22 2014 -0500
+++ b/Lib/test/test_inspect.py	Sat Mar 01 11:38:14 2014 -0500
@@ -1206,6 +1206,11 @@
         self.assertEqualException(f3, '1, 2')
         self.assertEqualException(f3, '1, 2, a=1, b=2')
 
+        # issue20817:
+        def f6(a, b, c):
+            pass
+        self.assertRaises(TypeError, inspect.getcallargs, f6)
+
 class TestGetcallargsMethods(TestGetcallargsFunctions):
 
     def setUp(self):