diff --git a/README.md b/README.md
index b41b8af..361480a 100644
--- a/README.md
+++ b/README.md
@@ -1085,6 +1085,7 @@ The proper way to have mutable default values is:
 #good:)
 def add_to(element, to=None):
     if to is None:
+        to = {}
         to['a'] = element
         return to
 ```