str.formatter invalid

lib/matplotlib/dates.py

@@ -707,7 +707,7 @@ def __call__(self, x, pos=None):
         elif six.callable(fmt):
             result = fmt(x, pos)
         else:
-            raise TypeError('Unexpected type passed to {!r}.'.formatter(self))
+            raise TypeError('Unexpected type passed to {0!r}.'.format(self))
 
         return result