Re-frame subs and spec.test/instrument

I have a normal re-frame subscription, e.g:

(require '[re-frame.core :refer [reg-sub]]
          '[orchestra-cljs.spec.test :as spec-test])

(s/fdef foo
  :ret int?)

(defn foo [] "fooo")

(reg-sub ::foo foo)

(spec-test/instrument)

The sub value gets dereffed in the component’s render fn, meaning that foo is definitely being called, but it doesn’t say anything about failing spec. If I call the function in the repl (by hand) - it fails the spec (as expected).

Does re-frame do anything special that the instrumented function gets ignored?