Confusing error when combining vector and non-vector monads

[codahale]

I have code which looks like this:

(use '[cats.builtin])
(mlet [a (just 10)
       b [1 2 3 4]]
      (return (+ a b)))

But throws this error

No implementation of method: :-repr of protocol: #'cats.protocols/Printable
   found for class: clojure.lang.PersistentVector

          core_deftype.clj:  568  clojure.core/-cache-protocol-fn
          core_deftype.clj:  560  clojure.core/-cache-protocol-fn
            protocols.cljc:   43  cats.protocols$eval23741$fn__23742$G__23732__23747/invoke
                maybe.cljc:  208  cats.monad.maybe$reify__24200/_mbind
                 core.cljc:   94  cats.core$bind/invokeStatic
                 core.cljc:   81  cats.core$bind/invoke

According to the docs, vector is (once builtin is required) a traversable. Am I holding it wrong, or is this a bug? Appreciate any help here.

[niwinz]

Hmm, the error is strange, but in any case, mixing monads in one mlet is not recommeded. In some cases it can work but it is not a way to go.