datomic.db spec error (https://github.com/Datomic/day-of-datomic)

error.txt

;; evaluating the hello-world namespace yields:

CompilerException clojure.lang.ExceptionInfo: Call to #'clojure.set/map-invert did not conform to spec:
db.clj:2708

-- Spec failed --------------------

Function arguments

  ({})
   ^^

should satisfy

  map?

or

  nil?

-------------------------
Detected 1 error
 {:clojure.spec.alpha/problems [{:path [:args :m :clojure.spec.alpha/pred], :pred clojure.core/map?, :val {}, :via [:flowthing.set.specs.alpha/nilable-map :flowthing.set.specs.alpha/nilable-map], :in [0]} {:path [:args :m :clojure.spec.alpha/nil], :pred nil?, :val {}, :via [:flowthing.set.specs.alpha/nilable-map :flowthing.set.specs.alpha/nilable-map], :in [0]}], :clojure.spec.alpha/spec #object[clojure.spec.alpha$regex_spec_impl$reify__2436 0x6f0c95da "clojure.spec.alpha$regex_spec_impl$reify__2436@6f0c95da"], :clojure.spec.alpha/value ({}), :clojure.spec.alpha/args ({}), :clojure.spec.alpha/failure :instrument, :clojure.spec.test.alpha/caller {:file "db.clj", :line 2708, :var-scope datomic.db/calc-tempids}}, compiling:(hello_world.clj:15:11)