diff --git a/pkgs/development/haskell-modules/lib/compose.nix b/pkgs/development/haskell-modules/lib/compose.nix
index f3a25e293d88..fe1a8ef7a014 100644
--- a/pkgs/development/haskell-modules/lib/compose.nix
+++ b/pkgs/development/haskell-modules/lib/compose.nix
@@ -354,7 +354,12 @@ rec {
   /* Add a dummy command to trigger a build despite an equivalent
      earlier build that is present in the store or cache.
    */
-  triggerRebuild = i: overrideCabal (drv: { postUnpack = ": trigger rebuild ${toString i}"; });
+  triggerRebuild = i: overrideCabal (drv: {
+    postUnpack = drv.postUnpack or "" + ''
+
+      # trigger rebuild ${toString i}
+    '';
+  });
 
   /* Override the sources for the package and optionally the version.
      This also takes of removing editedCabalFile.