If there is no nil values, why do I get null pointer exception?

[larry google groups]

I am getting a null pointer exception in the line where I conj into the 
vector. I added the pprint so I could see what was going on. I am confused 
by the outcome:

(defn convert-json-to-xml [json-as-flat-maps]
  (reduce
   (fn [vector-of-strings next-movie-as-map]
     (println "next move as map: ")
     (println (pp/pprint next-movie-as-map))
     (if (seq next-movie-as-map)
       (conj vector-of-strings (xml/emit-str (xml/map->Element 
next-movie-as-map)))
       vector-of-strings))
   []
   json-as-flat-maps))

The pprint is showing me this: 

{:film_64209.9096316473 "513e67e3c07f5dd745000051",
 :cast_member_64209.9096316473 "5148c50dc07f5db42300003a",
 :title_64209.9096316473 "Sound design",
 :director "",
 :runtime "5",
 :movie-id "513e67e3c07f5dd745000051",
 :title "Two Islands",
 :thumb
 
"https://s3.amazonaws.com/tribeca_cms_production/uploads/uploads/film/photo_1/513e67e3c07f5dd745000051/small_TWO_ISLANDS_2_pubs.jpg";,
 :categories ["Documentary"],
 :youtube_url "",
 :website_url "",
 :name_64209.9096316473 "Svante Colérus",
 :description
 "Two Islands is film about two enormous waste dumps in New York City. The 
first was once the largest dump in the world. The other, a cemetery of 
unknowns, is still in use. Two Islands bluntly asks, what does the 
existence of these two huge mountains of economic and social waste and 
rejected surplus tell us about our civilization and the so-called richest 
nation in the world? What kind of legacy will future archaeologists see?"}


Any thoughts about what triggers a null pointer exception? I am using this 
XML library: 

http://clojure.github.com/data.xml/






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