diff --git a/pkgs/development/haskell-modules/configuration-ghc-8.6.x.nix b/pkgs/development/haskell-modules/configuration-ghc-8.6.x.nix
index 1f76e8185a59..e87880f05cd5 100644
--- a/pkgs/development/haskell-modules/configuration-ghc-8.6.x.nix
+++ b/pkgs/development/haskell-modules/configuration-ghc-8.6.x.nix
@@ -46,6 +46,7 @@ self: super: {
 
   # LTS-12.x versions do not compile.
   base-orphans = self.base-orphans_0_8;
+  cassava-megaparsec = doJailbreak super.cassava-megaparsec;
   config-ini = doJailbreak super.config-ini;   # https://github.com/aisamanra/config-ini/issues/18
   contravariant = self.contravariant_1_5;
   free = self.free_5_1;