Nathan fails to understand something about assoc'ing onto an HMap with :optional keys

typetest.clj

(ns example.typetest
  (:use [clojure.core.typed]))
; using [org.clojure/core.typed "0.2.13"]

(def-alias House "a House has some :people in it and maybe a :mortgage."
  (HMap :mandatory {:id     String
                    :people (Coll String)
                    }
        ; comment out this :optional line and it all works
        :optional {:mortgage Number}
        ))

(ann my-house House)
(def my-house {:id "foo", :people ["Larry" "Curly" "Moe"]})

(comment
  (check-ns)  ;=> success

  (cf (assoc my-house :people ["Shemp"]) House)
  ;; this will return `House` if the :optional field above is commented out.
  ;;
  ;; but if the :optional field is left in the def-alias above, I get:
  ;Type Error (...) Cannot assoc args `[(Value :people) {:then tt, :else ff}] [[(Value "Shep")] {:then tt, :else ff}]` on typetest/House

  )