diff --git a/Makefile b/Makefile
index d9098d1..e63a1e6 100644
--- a/Makefile
+++ b/Makefile
@@ -1,7 +1,7 @@
 .PHONY : sdist clean
 
 language : src/Grammar/Test
-	cabal install --installdir=.
+	cabal install --installdir=. --overwrite-policy=always
 
 src/Grammar/Test.hs src/Grammar/Lex.x src/Grammar/Par.y : Grammar.cf
 	bnfc -o src -d $<
diff --git a/shell.nix b/shell.nix
index 2eaf1cd..84d3c04 100644
--- a/shell.nix
+++ b/shell.nix
@@ -1,5 +1,5 @@
 let
-  pkgs = import <nixpkgs> { }; # pin the channel to ensure reproducibility!
+  pkgs = import (fetchTarball https://github.com/NixOS/nixpkgs/archive/8c619a1f3cedd16ea172146e30645e703d21bfc1.tar.gz) { }; # pin the channel to ensure reproducibility!
 in
 pkgs.haskellPackages.developPackage {
   root = ./.;