diff --git a/pkgs/development/compilers/ocaml/4.01.0.nix b/pkgs/development/compilers/ocaml/4.01.0.nix
index 0f8ed7deaa4..0f68014fad6 100644
--- a/pkgs/development/compilers/ocaml/4.01.0.nix
+++ b/pkgs/development/compilers/ocaml/4.01.0.nix
@@ -1,12 +1,16 @@
-{ stdenv, fetchurl, ncurses, buildEnv, libX11, xproto,
-  useX11 ? (!stdenv.isArm && !stdenv.isMips) }:
+let
+  safeX11 = stdenv: !(stdenv.isArm || stdenv.isMips);
+in
+
+{ stdenv, fetchurl, ncurses, buildEnv, libX11, xproto, useX11 ? safeX11 stdenv }:
+
+if useX11 && !(safeX11 stdenv)
+  then throw "x11 not available in ocaml with arm or mips arch"
+  else # let the indentation flow
 
 let
    useNativeCompilers = !stdenv.isMips;
    inherit (stdenv.lib) optionals optionalString;
-   x11Ok = if useX11 && (!stdenv.isArm && !stdenv.isMips)
-           then throw "x11 not available in ocaml with arm or mips arch"
-           else true;
 in
 
 stdenv.mkDerivation rec {