diff --git a/lib/body.ml b/lib/body.ml
index dbca1c30..cdbc513f 100644
--- a/lib/body.ml
+++ b/lib/body.ml
@@ -37,13 +37,12 @@ type _ t =
   ; mutable write_final_if_chunked : bool
   ; mutable on_eof                 : unit -> unit
   ; mutable on_read                : Bigstringaf.t -> off:int -> len:int -> unit
-  ; mutable when_ready_to_write    : unit -> unit
+  ; mutable when_ready_to_write    : Optional_thunk.t
   ; buffered_bytes                 : int ref
   }
 
 let default_on_eof         = Sys.opaque_identity (fun () -> ())
 let default_on_read        = Sys.opaque_identity (fun _ ~off:_ ~len:_ -> ())
-let default_ready_to_write = Sys.opaque_identity (fun () -> ())
 
 let of_faraday faraday =
   { faraday
@@ -51,7 +50,7 @@ let of_faraday faraday =
   ; write_final_if_chunked = true
   ; on_eof                 = default_on_eof
   ; on_read                = default_on_read
-  ; when_ready_to_write    = default_ready_to_write
+  ; when_ready_to_write    = Optional_thunk.none
   ; buffered_bytes         = ref 0
   }
 
@@ -79,8 +78,8 @@ let schedule_bigstring t ?off ?len (b:Bigstringaf.t) =
 
 let ready_to_write t =
   let callback = t.when_ready_to_write in
-  t.when_ready_to_write <- default_ready_to_write;
-  callback ()
+  t.when_ready_to_write <- Optional_thunk.none;
+  Optional_thunk.unchecked_value callback ()
 
 let flush t kontinue =
   Faraday.flush t.faraday kontinue;
@@ -145,11 +144,11 @@ let close_reader t =
 ;;
 
 let when_ready_to_write t callback =
-  if not (t.when_ready_to_write == default_ready_to_write)
-  then failwith "Body.when_ready_to_write: only one callback can be registered at a time"
-  else if is_closed t
+  if is_closed t
   then callback ()
-  else t.when_ready_to_write <- callback
+  else if Optional_thunk.is_some t.when_ready_to_write 
+  then failwith "Body.when_ready_to_write: only one callback can be registered at a time"
+  else t.when_ready_to_write <- Optional_thunk.some callback
 
 let transfer_to_writer_with_encoding t ~encoding writer =
   let faraday = t.faraday in
diff --git a/lib/optional_thunk.ml b/lib/optional_thunk.ml
new file mode 100644
index 00000000..5a8cf961
--- /dev/null
+++ b/lib/optional_thunk.ml
@@ -0,0 +1,11 @@
+type t = unit -> unit
+
+let none = Sys.opaque_identity (fun () -> ())
+let some f =
+  if f == none
+  then failwith "Optional_thunk: this function is not representable as a some value";
+  f
+
+let is_none t = t == none
+let is_some t = not (is_none t)
+let unchecked_value t = t
diff --git a/lib/optional_thunk.mli b/lib/optional_thunk.mli
new file mode 100644
index 00000000..dd36f50d
--- /dev/null
+++ b/lib/optional_thunk.mli
@@ -0,0 +1,9 @@
+type t
+
+val none : t
+val some : (unit -> unit) -> t
+
+val is_none : t -> bool
+val is_some : t -> bool
+
+val unchecked_value : t -> unit -> unit
diff --git a/lib/reqd.ml b/lib/reqd.ml
index f598f5b5..87dbbea0 100644
--- a/lib/reqd.ml
+++ b/lib/reqd.ml
@@ -35,7 +35,7 @@ type error =
   [ `Bad_request | `Bad_gateway | `Internal_server_error | `Exn of exn ]
 
 type response_state =
-  | Waiting   of (unit -> unit) ref
+  | Waiting   of Optional_thunk.t ref
   | Complete  of Response.t
   | Streaming of Response.t * [`write] Body.t
 
@@ -78,8 +78,6 @@ type t =
   ; mutable error_code      : [`Ok | error ]
   }
 
-let default_waiting = Sys.opaque_identity (fun () -> ())
-
 let create error_handler request request_body writer response_body_buffer =
   { request
   ; request_body
@@ -87,14 +85,14 @@ let create error_handler request request_body writer response_body_buffer =
   ; response_body_buffer
   ; error_handler
   ; persistent              = Request.persistent_connection request
-  ; response_state          = Waiting (ref default_waiting)
+  ; response_state          = Waiting (ref Optional_thunk.none)
   ; error_code              = `Ok
   }
 
 let done_waiting when_done_waiting =
   let f = !when_done_waiting in
-  when_done_waiting := default_waiting;
-  f ()
+  when_done_waiting := Optional_thunk.none;
+  Optional_thunk.unchecked_value f ()
 
 let request { request; _ } = request
 let request_body { request_body; _ } = request_body
@@ -213,9 +211,9 @@ let error_code t =
 let on_more_output_available t f =
   match t.response_state with
   | Waiting when_done_waiting ->
-    if not (!when_done_waiting == default_waiting) then
-      failwith "httpaf.Reqd.on_more_output_available: only one callback can be registered at a time";
-    when_done_waiting := f
+    if Optional_thunk.is_some !when_done_waiting
+    then failwith "httpaf.Reqd.on_more_output_available: only one callback can be registered at a time";
+    when_done_waiting := Optional_thunk.some f
   | Streaming(_, response_body) ->
     Body.when_ready_to_write response_body f
   | Complete _ ->
diff --git a/lib/server_connection.ml b/lib/server_connection.ml
index fbaaa8b7..d57b5c44 100644
--- a/lib/server_connection.ml
+++ b/lib/server_connection.ml
@@ -64,12 +64,10 @@ type t =
   ; request_queue          : Reqd.t Queue.t
     (* invariant: If [request_queue] is not empty, then the head of the queue
        has already had [request_handler] called on it. *)
-  ; mutable wakeup_writer  : (unit -> unit)
-  ; mutable wakeup_reader  : (unit -> unit)
+  ; mutable wakeup_writer  : Optional_thunk.t
+  ; mutable wakeup_reader  : Optional_thunk.t
   }
 
-let default_wakeup = Sys.opaque_identity (fun () -> ())
-
 let is_closed t =
   Reader.is_closed t.reader && Writer.is_closed t.writer
 
@@ -85,35 +83,37 @@ let current_reqd_exn t =
 let yield_reader t k =
   if is_closed t
   then failwith "on_wakeup_reader on closed conn"
-  else if not (t.wakeup_reader == default_wakeup);
+  else if Optional_thunk.is_some t.wakeup_reader
   then failwith "yield_reader: only one callback can be registered at a time"
-  else t.wakeup_reader <- k
+  else t.wakeup_reader <- Optional_thunk.some k
 ;;
 
 let wakeup_reader t =
   let f = t.wakeup_reader in
-  t.wakeup_reader <- default_wakeup;
-  f ()
+  t.wakeup_reader <- Optional_thunk.none;
+  Optional_thunk.unchecked_value f ()
 ;;
 
 let on_wakeup_writer t k =
   if is_closed t
   then failwith "on_wakeup_writer on closed conn"
-  else if not (t.wakeup_writer == default_wakeup)
+  else if Optional_thunk.is_some t.wakeup_writer
   then failwith "yield_writer: only one callback can be registered at a time"
-  else t.wakeup_writer <- k
+  else t.wakeup_writer <- Optional_thunk.some k
 ;;
 
 let wakeup_writer t =
   let f = t.wakeup_writer in
-  t.wakeup_writer <- default_wakeup;
-  f ()
+  t.wakeup_writer <- Optional_thunk.none;
+  Optional_thunk.unchecked_value f ()
 ;;
 
 let transfer_writer_callback t reqd =
-  let f = t.wakeup_writer in
-  t.wakeup_writer <- default_wakeup;
-  Reqd.on_more_output_available reqd f
+  if Optional_thunk.is_some t.wakeup_writer
+  then (
+    let f = t.wakeup_writer in
+    t.wakeup_writer <- Optional_thunk.none;
+    Reqd.on_more_output_available reqd (Optional_thunk.unchecked_value f))
 ;;
 
 let default_error_handler ?request:_ error handle =
@@ -149,8 +149,8 @@ let create ?(config=Config.default) ?(error_handler=default_error_handler) reque
   ; request_handler = request_handler
   ; error_handler   = error_handler
   ; request_queue
-  ; wakeup_writer   = default_wakeup
-  ; wakeup_reader   = default_wakeup
+  ; wakeup_writer   = Optional_thunk.none
+  ; wakeup_reader   = Optional_thunk.none
   }
 
 let shutdown_reader t =