diff --git a/weakrefmethod.py b/weakrefmethod.py
index 1b8e73b..0836944 100644
--- a/weakrefmethod.py
+++ b/weakrefmethod.py
@@ -18,7 +18,7 @@ class WeakMethod(weakref.ref):
             obj = meth.__self__
             func = meth.__func__
         except AttributeError:
-            raise TypeError('argument should be a bound method, not {}'.format(type(meth)))
+            raise TypeError('argument should be a bound method, not {0}'.format(type(meth)))
 
         def _cb(arg):
             # The self-weakref trick is needed to avoid creating a reference cycle.