diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index 0eb56a8..3f83f65 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -3,7 +3,7 @@ name: build
 on:
   pull_request:
   push:
-    branch:
+    branches:
       - master
 
 jobs:
@@ -41,6 +41,7 @@ jobs:
       - name: Use OCaml ${{ matrix.ocaml-compiler }}
         uses: ocaml/setup-ocaml@v2
         with:
+          cache-prefix: v2
           ocaml-compiler: ${{ matrix.ocaml-compiler }}
           opam-depext-flags: --with-test
           opam-local-packages: |
diff --git a/Makefile b/Makefile
index a026454..437e303 100644
--- a/Makefile
+++ b/Makefile
@@ -34,9 +34,9 @@ clean:
 
 _opam:
 	opam switch create . --empty
-	opam switch set-invariant ocaml-base-compiler.4.14.1
 
 install-dependencies: _opam
+	opam switch set-invariant ocaml-base-compiler.5.1.1
 	opam install . --deps-only --with-test
 
 DOCS_WORKTREE_PATH=../ocaml-decoders-doc
diff --git a/dune-project b/dune-project
index ffb8f80..bf122b6 100644
--- a/dune-project
+++ b/dune-project
@@ -150,7 +150,7 @@
   (ocaml (>= 4.14.1))
   (odoc :with-doc)
   (decoders (= :version))
-   melange
+  (melange (>= 3.0.0))
   (melange-fetch :with-test)
   (melange-jest :with-test)
   (melange-webapi :with-test)))
diff --git a/melange-decoders.opam b/melange-decoders.opam
index 0bf3b0a..0397380 100644
--- a/melange-decoders.opam
+++ b/melange-decoders.opam
@@ -14,7 +14,7 @@ depends: [
   "ocaml" {>= "4.14.1"}
   "odoc" {with-doc}
   "decoders" {= version}
-  "melange"
+  "melange" {>= "3.0.0"}
   "melange-fetch" {with-test}
   "melange-jest" {with-test}
   "melange-webapi" {with-test}
diff --git a/src-js/js_json.ml b/src-js/js_json.ml
index ddbb981..b2547de 100644
--- a/src-js/js_json.ml
+++ b/src-js/js_json.ml
@@ -70,10 +70,10 @@ module Decode = struct
               let oks, errs =
                 arr
                 |> Js.Array.reducei
-                     (fun (oks, errs) x i ->
+                     ~f:(fun (oks, errs) x i ->
                        match decoder.dec x with
                        | Ok a ->
-                           let _ = Js.Array.push a oks in
+                           let _ = Js.Array.push ~value:a oks in
                            (oks, errs)
                        | Error e ->
                            let _ =
@@ -82,7 +82,7 @@ module Decode = struct
                                errs
                            in
                            (oks, errs) )
-                     ([||], [||])
+                     ~init:([||], [||])
               in
               if Js.Array.length errs > 0
               then
@@ -123,5 +123,5 @@ module Encode = struct
   include Encode.Make (Json_encodeable)
 
   let array encoder xs =
-    xs |> Js.Array.map (fun x -> encoder x) |> Js.Json.array
+    xs |> Js.Array.map ~f:(fun x -> encoder x) |> Js.Json.array
 end
diff --git a/src-js/js_xml.ml b/src-js/js_xml.ml
index 7820661..fe6ee78 100644
--- a/src-js/js_xml.ml
+++ b/src-js/js_xml.ml
@@ -12,7 +12,7 @@ module DOMParser = struct
     [@@mel.get]
 
   external querySelector :
-    Dom.element -> string -> Dom.element Js.null_undefined = "querySelector"
+    Dom.element -> string -> Dom.element Js.nullable = "querySelector"
     [@@mel.send]
 
   external textContent : Dom.element -> string = "textContent" [@@mel.get]
@@ -96,7 +96,7 @@ module Element = struct
 
 
   external append : Dom.element -> Dom.node array -> unit = "append"
-    [@@mel.send] [@@variadic]
+    [@@mel.send] [@@mel.variadic]
 
   external setAttribute : Dom.element -> string -> string -> unit
     = "setAttribute"
@@ -114,10 +114,10 @@ end
 
 module Document = struct
   external createElementNS : string -> string -> Dom.element = "createElementNS"
-    [@@scope "window", "document"]
+    [@@mel.scope "window", "document"]
 
   external createTextNode : string -> Dom.text = "createTextNode"
-    [@@scope "window", "document"]
+    [@@mel.scope "window", "document"]
 end
 
 module Encode = struct