diff --git a/lwt-unix/httpaf_lwt_unix.ml b/lwt-unix/httpaf_lwt_unix.ml
index 9dc7f1db..5fa579f4 100644
--- a/lwt-unix/httpaf_lwt_unix.ml
+++ b/lwt-unix/httpaf_lwt_unix.ml
@@ -171,6 +171,7 @@ module Server = struct
       ?(config=Config.default)
       ~request_handler
       ~error_handler =
+      let make_tls_server = Tls_io.make_server ?server ?certfile ?keyfile in
       fun client_addr socket ->
         let connection =
           Server_connection.create
@@ -178,7 +179,7 @@ module Server = struct
             ~error_handler:(error_handler client_addr)
             (request_handler client_addr)
         in
-        Tls_io.make_server ?server ?certfile ?keyfile socket >>= fun tls_server ->
+        make_tls_server socket >>= fun tls_server ->
         let readf = Tls_io.readf tls_server in
         let writev = Tls_io.writev tls_server in
         start_read_write_loops ~config ~readf ~writev ~socket connection
@@ -193,6 +194,7 @@ module Server = struct
       ?(config=Config.default)
       ~request_handler
       ~error_handler =
+      let make_ssl_server = Ssl_io.make_server ?server ?certfile ?keyfile in
       fun client_addr socket ->
         let connection =
           Server_connection.create
@@ -200,7 +202,7 @@ module Server = struct
             ~error_handler:(error_handler client_addr)
             (request_handler client_addr)
         in
-        Ssl_io.make_server ?server ?certfile ?keyfile socket >>= fun tls_server ->
+        make_ssl_server socket >>= fun tls_server ->
         let readf = Ssl_io.readf tls_server in
         let writev = Ssl_io.writev tls_server in
         start_read_write_loops ~config ~readf ~writev ~socket connection
diff --git a/lwt-unix/ssl_io_dummy.ml b/lwt-unix/ssl_io_dummy.ml
index eb56f5ee..e438aa0e 100644
--- a/lwt-unix/ssl_io_dummy.ml
+++ b/lwt-unix/ssl_io_dummy.ml
@@ -9,8 +9,10 @@ let writev _socket _fd =
 type client = [ `Ssl_not_available  ]
 type server = [ `Ssl_not_available  ]
 
-let make_client ?client:_ _socket =
-  Lwt.fail_with "Ssl not available"
+let[@ocaml.warning "-21"] make_client ?client:_ =
+  failwith "Ssl not available";
+  fun _socket -> Lwt.fail_with "Ssl not available"
 
-let make_server ?server:_ ?certfile:_ ?keyfile:_ _socket =
-  Lwt.fail_with "Ssl not available"
+let[@ocaml.warning "-21"] make_server ?server:_ ?certfile:_ ?keyfile:_ =
+  failwith "Ssl not available";
+  fun _socket -> Lwt.fail_with "Ssl not available"
diff --git a/lwt-unix/tls_io_dummy.ml b/lwt-unix/tls_io_dummy.ml
index d3eb8f23..231458db 100644
--- a/lwt-unix/tls_io_dummy.ml
+++ b/lwt-unix/tls_io_dummy.ml
@@ -9,8 +9,10 @@ let writev _tls _fd =
 type client = [ `Tls_not_available ]
 type server = [ `Tls_not_available ]
 
-let make_client ?client:_ _socket =
-  Lwt.fail_with "Tls not available"
+let[@ocaml.warning "-21"] make_client ?client:_ =
+  failwith "TLS not available";
+  fun _socket -> Lwt.return `Tls_not_available
 
-let make_server ?server:_ ?certfile:_ ?keyfile:_ _socket =
-  Lwt.fail_with "Tls not available"
+let[@ocaml.warning "-21"] make_server ?server:_ ?certfile:_ ?keyfile:_ =
+  failwith "TLS not available";
+  fun _socket -> Lwt.fail_with "TLS not available"