diff --git a/flake.nix b/flake.nix
index 5883103..56615cf 100644
--- a/flake.nix
+++ b/flake.nix
@@ -236,7 +236,7 @@
             ];
 
             shellHook = ''
-              if [ -z "''${SKIP_MOD_WORKTREES}" ]; then
+              if [ -z "''${SKIP_MOD_WORKTREES:-}" ]; then
                 mod_worktrees
               fi
             '';