diff --git a/pkgs/development/lisp-modules/README.txt b/pkgs/development/lisp-modules/README.txt
index 08452d979ad..46d7208bcd2 100644
--- a/pkgs/development/lisp-modules/README.txt
+++ b/pkgs/development/lisp-modules/README.txt
@@ -1,6 +1,6 @@
 Want to add a package?  There are 3 simple steps!
 1. Add the needed system names to quicklisp-to-nix-systems.txt.
-2. cd <path to quicklisp-to-nix-systems.txt> ; nix-shell --run 'quicklisp-to-nix .'
+2. cd <path to quicklisp-to-nix-systems.txt> ; nix-shell --pure --run 'quicklisp-to-nix .'
   You might want to specify also the --cacheSystemInfoDir and --cacheFaslDir
   parameters to preserve some data between runs. For example, it is very
   useful when you add new packages with native dependencies and fail to