diff --git a/bin/builder_db_app.ml b/bin/builder_db_app.ml
index 2eb6c9c..b5e3abb 100644
--- a/bin/builder_db_app.ml
+++ b/bin/builder_db_app.ml
@@ -892,7 +892,7 @@ let default_cmd, default_info =
   Cmd.info ~doc "builder-db"
 
 let () =
-  Mirage_crypto_rng_unix.initialize ();
+  Mirage_crypto_rng_unix.initialize (module Mirage_crypto_rng.Fortuna);
   Cmdliner.Cmd.group
     ~default:default_cmd default_info
     [ help_cmd; migrate_cmd;
diff --git a/builder-web.opam b/builder-web.opam
index c7c4273..22a675b 100644
--- a/builder-web.opam
+++ b/builder-web.opam
@@ -22,12 +22,12 @@ depends: [
   "cstruct" {>= "6.0.0"}
   "bos"
   "hex"
-  "lwt" {>= "5.6.0"}
+  "lwt" {>= "5.7.0"}
   "caqti" {>= "1.8.0"}
   "caqti-lwt"
   "caqti-driver-sqlite3"
   "pbkdf"
-  "mirage-crypto-rng"
+  "mirage-crypto-rng" {>= "0.11.0"}
   "scrypt-kdf"
   "opam-core"
   "opam-format" {>= "2.1.0"}
diff --git a/lib/builder_web.ml b/lib/builder_web.ml
index 5ed367b..8b72c79 100644
--- a/lib/builder_web.ml
+++ b/lib/builder_web.ml
@@ -247,7 +247,7 @@ module Viz_aux = struct
      |> Lwt.return
      |> if_error "Error finding a version of the requested visualization")
     >>= fun viz_path ->
-    Lwt_result.catch (
+    Lwt_result.catch (fun () ->
       Lwt_io.with_file ~mode:Lwt_io.Input
         (Fpath.to_string viz_path)
         Lwt_io.read
diff --git a/lib/dream_tar.ml b/lib/dream_tar.ml
index fc58812..b5a5899 100644
--- a/lib/dream_tar.ml
+++ b/lib/dream_tar.ml
@@ -15,7 +15,7 @@ module Writer = struct
       | `Await gz -> state.gz <- gz ; Lwt.return_unit
       | `Flush gz ->
         let max = Cstruct.length oc - Gz.Def.dst_rem gz in
-        let str = Cstruct.copy oc 0 max in
+        let str = Cstruct.to_string ~len:max oc in
         Dream.write stream str >>= fun () ->
         let { Cstruct.buffer; off= cs_off; len= cs_len; } = oc in
         until_await (Gz.Def.dst gz buffer cs_off cs_len)
@@ -76,7 +76,7 @@ let targz_response datadir finish (files : Builder_db.file list) (stream : Dream
     | `Await _gz -> assert false
     | `Flush gz | `End gz as flush_or_end ->
       let max = Cstruct.length state.oc - Gz.Def.dst_rem gz in
-      let str = Cstruct.copy state.oc 0 max in
+      let str = Cstruct.to_string ~len:max state.oc in
       Dream.write stream str >>= fun () -> match flush_or_end with
       | `Flush gz ->
         let { Cstruct.buffer; off= cs_off; len= cs_len; } = state.oc in
diff --git a/test/test_builder_db.ml b/test/test_builder_db.ml
index 66acbe3..41d8b1a 100644
--- a/test/test_builder_db.ml
+++ b/test/test_builder_db.ml
@@ -3,7 +3,7 @@ let ( >>| ) x f = Result.map f x
 
 module type CONN = Caqti_blocking.CONNECTION
 
-let () = Mirage_crypto_rng_unix.initialize ()
+let () = Mirage_crypto_rng_unix.initialize (module Mirage_crypto_rng.Fortuna)
 
 let iter f xs = List.fold_left (fun r x -> r >>= fun () -> f x) (Ok ()) xs
 let get_opt message = function