cljs-time null error?

gistfile1.txt

clj-demgen.main> (cemerick.piggieback/cljs-repl (cljs.repl.rhino/repl-env))
To quit, type: :cljs/quit
nil
cljs.user> (require '[cljs-time.core :as date])
nil
cljs.user> (require '[cljs-time.format :as date-format])j
nil
cljs.user> (date-format/unparse (date-format/formatters :basic-date-time)
                                 (date/today-at 12 00))
org.mozilla.javascript.EcmaError: TypeError: Cannot call method "call" of null (rhino.clj#41)