Nil value in heterogeneous tuple throws a NullPointerException

Datomic Cloud
1

nil is a valid heterogeneous tuple value, right?

Given the following schema:

{:db/ident       :person/name
 :db/valueType   :db.type/string
 :db/cardinality :db.cardinality/one}

{:db/ident       :person/parents
 :db/valueType   :db.type/tuple
 :db/tupleTypes  [:db.type/ref :db.type/ref]
 :db/cardinality :db.cardinality/one}

I can happily transact:

{:tx-data [{:db/id          "child"
            :person/name    "child"
            :person/parents ["parent-a" nil]}]}
=> Success

And happily pull the result:

(d/q '{:find  [(pull ?p [*])]
       :in    [$]
       :where [[?p :person/name "child"]]}
     db) 

=> [[{:db/id          101155069755487
      :person/name    "child"
      :person/parents [101155069755485 nil]}]]

However, when attempting to untuple the tuple values and bind to their values, the binding to the nil value throws a NullPointerException:

(d/q '{:find  [?person ?parent-a-name ?parent-b-name]
       :in    [$]
       :where [[?person :person/name "child"]
               [?person :person/parents ?parents-tuple]
               [(untuple ?parents-tuple) [?parent-a ?parent-b]]
               [?parent-a :person/name ?parent-a-name]
               [?parent-b :person/name ?parent-b-name] ;<-- adding this throws exception
               ]}
     db)

Execution error (NullPointerException) at datomic.core.db/asserting-datum (db.clj:-1).
null
clojure.lang.ExceptionInfo: processing clause: [?parent-b :person/name ?parent-b-name], message:  #:cognitect.anomalies{:category :cognitect.anomalies/incorrect, :message "processing clause: [?parent-b :person/name ?parent-b-name], message: "}
	at datomic.core.datalog$throw_query_ex_BANG_.invokeStatic(datalog.clj:50)
java.lang.NullPointerException

Is this expected? Thanks!