diff --git a/pkgs/top-level/haskell-defaults.nix b/pkgs/top-level/haskell-defaults.nix
index 9ff9c91aacc..fb82245344c 100644
--- a/pkgs/top-level/haskell-defaults.nix
+++ b/pkgs/top-level/haskell-defaults.nix
@@ -83,6 +83,7 @@
   ghc6104Prefs =
     self : self.haskellPlatformArgs_2009_2_0_2 self // {
       haskellPlatform = self.haskellPlatform_2009_2_0_2;
+      mtl = self.mtl_1_1_0_2;
       mtl1 = self.mtl_1_1_0_2;
       extensibleExceptions = self.extensibleExceptions_0_1_1_0;
       text = self.text_0_11_0_6;