以下Clojure协议有什么问题？

(defprotocol PP 
  (foo [bar]) 
  (foo [bar baz]))
=> PP

(extend-protocol PP 
  Object 
    (foo [bar] 1) 
    (foo [bar baz] 2))
=> nil

(foo "hello!")
=> #<CompilerException java.lang.IllegalArgumentException: No single method: foo of interface: PP found for function: foo of protocol: PP

我哪里错了？作为foo函数的单参数版本的结果，我希望看到1，因为“hello！”显然是java.lang.Object的一个实例。

user> (defprotocol PP
        (foo [bar] [bar baz]))
PP
user> (extend-protocol PP
        Object
        (foo 
          ([bar] 1)
          ([bar baz] 2)))
nil
user> (foo "foo")
1
user> (foo "foo" "bar")
2