Trying to create a print-method for datomic.db.Datum

I am trying to create a print-method for datomic.db.Datum but it throws a ClassNotFoundException.

(def datum-elements (juxt :e :a :v :tx :added))

;; Removed because of ClassNotFoundException
(defmethod print-method datomic.db.Datum [v ^java.io.Writer w]
  (print-method "datomic.db.Datum" w)
  (print-method (datum-elements v) w))

“Type”: “java.lang.ClassNotFoundException”,
“Message”: “datomic.db.Datum”,
“At”: [
“java.net.URLClassLoader”,
“findClass”,
“URLClassLoader.java”,
382