diff --git a/modules/nixos.nix b/modules/nixos.nix
index e45e8e0..ad76729 100644
--- a/modules/nixos.nix
+++ b/modules/nixos.nix
@@ -1,6 +1,6 @@
 { config, pkgs, lib, ... }: with lib; let
   cfg = config.services.ssh-cert-dist;
-  ca = if builtins.isPath cfg.ca then cfg.ca else pkgs.writeText "ssh-ca" cfg.ca;
+  ca = if isStorePath cfg.ca then cfg.ca else pkgs.writeText "ssh-ca" cfg.ca;
 in
 {
   options.services.ssh-cert-dist = {