Transit problems with msgpack

[Casper]

So I have some code that is acting up when using msgpack. I have made a minimal example below, that shows the problem. The code returns -17 (long) when using :msgpack, but decodes correctly when using :json.

My impression was that :msgpack was base64 encoded and could be represented as a string (and logged and so on). Is that not the case, or is something else messing me up when I slurp the ByteArrayInputStream?

Code:

(require '[cognitect.transit :as transit])

(import [java.io ByteArrayInputStream ByteArrayOutputStream])

;; Write data to a stream

(def out (ByteArrayOutputStream.))

(transit/write (transit/writer out :msgpack) {:a [1 2]})

;; Read data from a stream -> string via slurp -> back to InputStream

(def string (slurp (ByteArrayInputStream. (.toByteArray out))))

(prn "Encoded string" string)

(def in (ByteArrayInputStream. 

          (.getBytes string "UTF-8")))

(def reader (transit/reader in :msgpack))

(transit/read reader)

Any idea what is going on? 

/Casper