StackOverflowError in Clojure

(defn my-fn [a b]
  (swap! a #(assoc % :out [b]))
  (swap! b #(assoc % :in [a])))
(my-fn (atom {}) (atom {}))

(clojure.stacktrace/e)

outputs this:

java.lang.StackOverflowError: null
 at clojure.lang.ArraySeq.<init> (ArraySeq.java:57)
    clojure.lang.ArraySeq.next (ArraySeq.java:81)
    clojure.lang.PersistentHashMap.create (PersistentHashMap.java:76)
    clojure.core$hash_map.doInvoke (core.clj:365)
    clojure.lang.RestFn.invoke (RestFn.java:421)
    clojure.core$print_sequential.invoke (core_print.clj:41)
    clojure.core/fn (core_print.clj:194)
    clojure.lang.MultiFn.invoke (MultiFn.java:231)