parameterized query with lookup ref returns different result to inlined lookup ref

There's a bunch of similarly looking issues from the past but none seem to be this particular case. A bad lookup ref inside the query throws an exception but the same query returns an empty set when the lookup ref is passed in as a parameter. Since the resulting query is identical I'd expect the semantics to be identical too.

% clj -Sdeps '{:deps {datascript {:mvn/version "0.18.8"}}}'
Downloading: datascript/datascript/0.18.8/datascript-0.18.8.pom from https://repo.clojars.org/
Downloading: datascript/datascript/0.18.8/datascript-0.18.8.jar from https://repo.clojars.org/
Clojure 1.9.0
user=> (require '[datascript.core :as db])
nil
user=> (def s {:id {:db/unique :db.unique/identity}})
#'user/s
user=> (def c (db/create-conn s))
#'user/c
user=> (db/transact c [{:id :a}])
#object[datascript.core$transact$reify__3612 0x7f2c995b {:status :ready, :val #datascript.db.TxReport{:db-before #datascript/DB {:schema {:id #:db{:unique :db.unique/identity}}, :datoms []}, :db-after #datascript/DB {:schema {:id #:db{:unique :db.unique/identity}}, :datoms [[1 :id :a 536870913]]}, :tx-data [#datascript/Datom [1 :id :a 536870913 true]], :tempids #:db{:current-tx 536870913}, :tx-meta nil}}]
user=> (db/q '[:find ?id :where [[:id :b] :id ?id]] @c)
ExceptionInfo Nothing found for entity id [:id :b]  clojure.core/ex-info (core.clj:4739)
user=> (db/q '[:find ?id :in $ ?idsel :where [?idsel :id ?id]] @c [:id :b])
#{}

Start by running the supplied Clojure reproduction against Datascript 0.18.8 and compare the inlined lookup ref with the parameterized form. Trace the query evaluation paths for both cases; done means an invalid lookup ref has consistent semantics and the behavior is covered by a regression test.