Yea, that's what one would think, but then it gives a type error:
```
File "src/paxos/multi_paxos.ml", line 66, characters 4-38:
Error: This expression has type ?sub_target:string -> unit Lwt.t
       but an expression was expected of type 'a Lwt.t
Command exited with code 2.
```
Passing in 'None' was the easiest solution