diff --git a/pgx_lwt/src/pgx_lwt.ml b/pgx_lwt/src/pgx_lwt.ml
index 70f827d..b44d836 100644
--- a/pgx_lwt/src/pgx_lwt.ml
+++ b/pgx_lwt/src/pgx_lwt.ml
@@ -47,6 +47,7 @@ module Thread = struct
 
     let close_in = Io.close_in
     let open_connection = Io.open_connection
+    let upgrade_ssl = `Not_supported
     let getlogin = Io.getlogin
     let debug s = Logs_lwt.debug (fun m -> m "%s" s)
     let protect f ~finally = Lwt.finalize f finally
diff --git a/pgx_unix/src/pgx_unix.ml b/pgx_unix/src/pgx_unix.ml
index d0b94b6..501ceb4 100644
--- a/pgx_unix/src/pgx_unix.ml
+++ b/pgx_unix/src/pgx_unix.ml
@@ -55,6 +55,8 @@ module Simple_thread = struct
     Unix.open_connection std_socket
   ;;
 
+  let upgrade_ssl = `Not_supported
+
   let output_char = output_char
   let output_binary_int = output_binary_int
   let output_string = output_string