Trying to figure out why line 13 is throwing an `ArityException` instead of yielding `"hello world"`

defprotocol_ArityException.clj

(defn slice
  ([]
    (slice "hello world"))
  ([v]
   (-slice v)))

(defprotocol Pizza
  (-slice [this]))

(extend-type String
  Pizza
  (-slice [this] (do this)))

(slice "works") ;; works
(slice) ;; hello world

Fixed with @gfredericks help.