name_that_nil.clj

(def scenes [{:subject  "Frankie"
              :action   "say"
              :object   "relax"}

             {:subject  "Lucy"
              :action   "❤s"
              :object   "Clojure"}

             {:subject  "Rich"
              :action   "tries"
              :object   "a new conditioner"}])

(defn people-in-scenes [scenes]
  (->> scenes
       (map :bad-key)  ;; <- nil is introduced here
       (interpose ", ")
       (reduce str)))


(println "People:" (people-in-scenes scenes))
;;=> People: , , 
;;=> nil

Response to https://twitter.com/swannodette/status/555541046537383938