NullPointerException in Laser library

; <ul id="mylist">
;  <li id="item1"><a href="link1">1</a></li>
; </ul>
(def html
  (l/parse "<ul id=\"mylist\"><li id=\"item1\"><a href=\"link1\">1</a></li></ul>"))

; Transformer function for the <ul>. This would do real work in the actual
; app, but here we just want to see if the "select" successfully located the
; first item in the list.
(defn mess-with-ul [ul]
  (let [list-item (l/select ul (l/id= "item1"))]
    (println list-item)))

; What's wrong with my API usage?
webapp.layout=> (l/document html (l/id= "mylist") mess-with-ul)
NullPointerException   clojure.zip/branch? (zip.clj:73)