diff --git a/pkgs/development/libraries/haskell/HTTP/4000.2.13.nix b/pkgs/development/libraries/haskell/HTTP/4000.2.13.nix
index 3a78e8f1674..9963b7e9770 100644
--- a/pkgs/development/libraries/haskell/HTTP/4000.2.13.nix
+++ b/pkgs/development/libraries/haskell/HTTP/4000.2.13.nix
@@ -14,6 +14,9 @@ cabal.mkDerivation (self: {
   ];
   jailbreak = true;
   doCheck = false;
+  preConfigure = self.stdenv.lib.optional (self.stdenv.lib.versionOlder "7.9" self.ghc.version) ''
+    sed -i -e 's|Extensions: |Extensions: FlexibleContexts, |' HTTP.cabal
+  '';
   meta = {
     homepage = "https://github.com/haskell/HTTP";
     description = "A library for client-side HTTP";