Problem with proxy of PersistentArrayMap

proxy_map.clj

   (defn with-fallback [amap fallback-fn]
     (proxy [clojure.lang.PersistentArrayMap
             clojure.lang.IFn]
         [(into-array Object amap)]
       (invoke [key] (or (get amap key)
                         (fallback-fn amap key)))))

   (def a (with-fallback {:a 1}
            (fn [amap key] (str "no value for " key " in " amap))))

   (a :a) ;=> 1
   (a :b) ;=> "no value for :b in {:a 1}"
   
   (assoc a :c 2)
   ;but this yields a "Unhandled java.lang.ArrayIndexOutOfBoundsException"