Error when compiling https://github.com/struktured/ocaml-mas/blob/broker-environment/src/environments/async/agent_pipe.ml

ocaml build errror

+ /home/cpiccion/.opam/4.02.1/bin/ocamlfind ocamlc -c -g -annot -bin-annot -I src/core -I src/core/async -I src/value_functions/async -package async -package containers -package containers.misc -package gen -package oml -package ppx_deriving -package ppx_deriving.iter -package ppx_deriving.ord -package ppx_deriving.show -package ppx_deriving_protobuf -package prob_cache_containers -package threads -thread -I src/environments/async -I src/core/async -I src/value_functions/async -o src/environments/async/agent_pipe.cmo src/environments/async/agent_pipe.ml
File "src/environments/async/agent_pipe.ml", line 43, characters 32-39:
Error: This expression has type
         ('a, 'b) Mas_core_async.Mas_async_system.Observation.t
         Async.Std.Pipe.Writer.t =
           (('a, 'b) Mas_core_async.Mas_async_system.Observation.t,
            Async.Std.Pipe.Writer.phantom)
           Async_kernel.Pipe.t
       but an expression was expected of type
         'b Async.Std.Pipe.Writer.t =
           ('b, Async.Std.Pipe.Writer.phantom) Async_kernel.Pipe.t
       The type variable 'b occurs inside
       ('a, 'b) Mas_core_async.Mas_async_system.Observation.t
Command exited with code 2.