diff --git a/pkgs/development/ocaml-modules/camlpdf/default.nix b/pkgs/development/ocaml-modules/camlpdf/default.nix
index 01441fcb4ec..a0a00472a4d 100644
--- a/pkgs/development/ocaml-modules/camlpdf/default.nix
+++ b/pkgs/development/ocaml-modules/camlpdf/default.nix
@@ -21,6 +21,12 @@ stdenv.mkDerivation rec {
     EOF
   '';
 
+  makeFlags = with stdenv.lib;
+  optionals (versionAtLeast ocaml.version "4.06") [
+    "OCAMLBCFLAGS+=-unsafe-string"
+    "OCAMLNCFLAGS+=-unsafe-string"
+  ];
+
   createFindlibDestdir = true;
 
   meta = with stdenv.lib; {
diff --git a/pkgs/development/ocaml-modules/cpdf/default.nix b/pkgs/development/ocaml-modules/cpdf/default.nix
index bcbb96c2c0c..b6473126158 100644
--- a/pkgs/development/ocaml-modules/cpdf/default.nix
+++ b/pkgs/development/ocaml-modules/cpdf/default.nix
@@ -14,6 +14,12 @@ stdenv.mkDerivation {
   buildInputs = [ ocaml findlib ncurses ];
   propagatedBuildInputs = [ camlpdf ];
 
+  makeFlags = with stdenv.lib;
+  optionals (versionAtLeast ocaml.version "4.06") [
+    "OCAMLBCFLAGS+=-unsafe-string"
+    "OCAMLNCFLAGS+=-unsafe-string"
+  ];
+
   createFindlibDestdir = true;
 
   postInstall = ''