diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index 69ce2af..7342c66 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -13,22 +13,30 @@ concurrency:
 
 jobs:
   build:
-    runs-on: ${{ matrix.operating-system }}
+    runs-on: ubuntu-latest
     strategy:
       matrix:
-        operating-system: [ubuntu-latest]
-        ocaml-version: ['4.09.0', '4.14.1', '5.0.0']
+        setup:
+          - {ocaml-version: '4.03', runtest: false}
+          - {ocaml-version: '4.09', runtest: true}
+          - {ocaml-version: '4.14', runtest: true}
+          - {ocaml-version: '5.0', runtest: true}
     steps:
     - uses: actions/checkout@v2
     - uses: ocaml/setup-ocaml@v2
       with:
-        ocaml-compiler: ${{ matrix.ocaml-version }}
+        ocaml-compiler:  ${{ matrix.setup.ocaml-version }}
     - name: Setup opam
       run: opam pin add -n .
     - name: Install dependencies
       run: opam depext -yt mad
+    - name: Build
+      if: ${{ ! matrix.setup.runtest }}
+      run: opam install .
     - name: Build and test
+      if: ${{ matrix.setup.runtest }}
       run: opam install -t .
+
   nix-build:
     runs-on: ${{ matrix.setup.os }}
     strategy:
diff --git a/dune-project b/dune-project
index 5dc002c..2667e48 100644
--- a/dune-project
+++ b/dune-project
@@ -1,2 +1,3 @@
 (lang dune 2.7)
+
 (name ssl)
diff --git a/src/ssl.ml b/src/ssl.ml
index 1f29659..44e2721 100644
--- a/src/ssl.ml
+++ b/src/ssl.ml
@@ -90,19 +90,25 @@ module Error = struct
 
   let get_error () =
     error_struct Get_error
-  
+
   let peek_error () =
     error_struct Peek_error
-  
+
   let peek_last_error () =
     error_struct Peek_last_error
 
   (** Reproduces the string format from ERR_error_string_n *)
   let peek_last_error_string () =
     let err = peek_last_error () in
-    let libstring = Option.value err.lib ~default:"lib(0)" in
-    let reasonstring = Option.value err.reason ~default:"reason(0)" in
-    Printf.sprintf "error:%08lX:%s::%s" (Int32.of_int err.code) libstring reasonstring
+    let libstring = match err.lib with Some lib -> lib | None -> "lib(0)" in
+    let reasonstring =
+      match err.reason with Some reason -> reason | None -> "reason(0)"
+    in
+    Printf.sprintf
+      "error:%08lX:%s::%s"
+      (Int32.of_int err.code)
+      libstring
+      reasonstring
 end
 
 exception Method_error
diff --git a/src/ssl_stubs.c b/src/ssl_stubs.c
index 418ba21..875d744 100644
--- a/src/ssl_stubs.c
+++ b/src/ssl_stubs.c
@@ -644,7 +644,15 @@ CAMLprim value ocaml_ssl_digest(value vevp, value vcert)
       caml_raise_with_arg(*caml_named_value("ssl_exn_certificate_error"), caml_copy_string(buf));
     }
   vdigest = caml_alloc_string(digest_size);
-  memcpy(Bytes_val(vdigest), buf, digest_size);
+
+  /* TODO(anmonteiro): switch this to `Bytes_val` when we bump support to
+   * OCaml 4.06 (https://github.com/ocaml/ocaml/pull/1274)
+   *
+   * In the meantime, reproduce `Bytes_val`, which is effectively `String_val`
+   * + a cast:
+   * https://github.com/ocaml/ocaml/pull/1274/commits/6bc4f2656e435175188018830e7fe049caacebe9
+   */
+  memcpy((unsigned char *)String_val(vdigest), buf, digest_size);
   CAMLreturn(vdigest);
 }
 
diff --git a/ssl.opam b/ssl.opam
index f70608d..4341269 100644
--- a/ssl.opam
+++ b/ssl.opam
@@ -14,8 +14,8 @@ run-test: [
   ["dune" "runtest" "-p" name "-j" jobs]
 ]
 depends: [
-  "ocaml" {>= "4.02.0"}
-  "dune" {>= "2.0.7"}
+  "dune" {>= "2.7"}
+  "ocaml" {>= "4.03.0"}
   "dune-configurator"
   "base-unix"
   "conf-libssl"