Type error: `Eof isn't the same as `Eof ?

I have this code that tries to log the output of a pipe:
 

   Async.Std.print_string (match (Async.Std.Pipe.read r) with
                           | `Eof -> "an empty string"
                           | `Ok text -> text)

 

 However, I get this type error when compiling:
 

 File "hello_world.ml", line 57, characters 29-33:
Error: This pattern matches values of type [? `Eof ]
       but a pattern was expected which matches values of type
         [ `Eof | `Ok of string ] Async_conduit.io

 

 Should I qualify these `Eof and `Ok somehow?

 

 What I am trying to do is to log the content of the body of a POST request. The body content is to be salvaged through a pipe.
 

 The full code can be seen here:

 https://github.com/loldrup/ocaml-cohttp/blob/master/examples/async/hello_world.ml https://github.com/loldrup/ocaml-cohttp/blob/master/examples/async/hello_world.ml