How to solve this type problem?

[yusdacra]

Hello, new to koka here. I'm really liking the language, and was trying to implement a simple async-await future type. So basically my question is related to the code over here on my repository https://github.com/yusdacra/koka_playground/blob/master/async/utils.kk

Basically what I wanted to do was add select and join functions for two futures. However I am facing problems with it as I get this error (when select and join are uncommented):

async/utils.kk(67,28): error: abstract types do not match
  context      :                            stt.fut2
  term         :                            stt
  inferred type: optional<ref<global,async/core/future-state<$a,$b>>>
  expected type: optional<ref<global,async/core/future-state<$c,$d>>>
  hint         : an higher-rank type escapes its scope?

I have tried many stuff but I couldn't get it to work it. I'm not sure as to why the types don't match. Perhaps I could have another version by defining my own effect(s) but I wanted to try this first.