diff --git a/pkgs/development/libraries/haskell/utf8-string/default.nix b/pkgs/development/libraries/haskell/utf8-string/default.nix
index ec946dcdbdf3..1233ce8806d3 100644
--- a/pkgs/development/libraries/haskell/utf8-string/default.nix
+++ b/pkgs/development/libraries/haskell/utf8-string/default.nix
@@ -4,6 +4,10 @@ cabal.mkDerivation (self: {
   pname = "utf8-string";
   version = "0.3.8";
   sha256 = "1h29dn0scsfkhmkg14ywq9178lw40ah1r36w249zfzqr02y7qxc0";
+  noHaddock = self.stdenv.lib.versionOlder self.ghc.version "6.11";
+  patchPhase = ''
+    sed -ir -e 's|Extensions: | Extensions: UndecidableInstances, |' utf8-string.cabal
+  '';
   meta = {
     homepage = "http://github.com/glguy/utf8-string/";
     description = "Support for reading and writing UTF8 Strings";