diff --git a/modules/default.nix b/modules/default.nix
index b316c70..cd44080 100644
--- a/modules/default.nix
+++ b/modules/default.nix
@@ -127,6 +127,7 @@ in
   config = lib.mkIf cfg.enable {
 
     environment.systemPackages = [ pkgs.tvix ];
+    users.users.tvix-castore = { };
     users.groups.tvix-castore = { };
 
     systemd.tmpfiles.rules = [ "d ${cfg.castoreDir} 770 root tvix-castore -" ];
@@ -143,7 +144,7 @@ in
             ExecStart = "${pkgs.tvix}/bin/tvix-store --otlp=false daemon --listen-address=\"${cache.grpcListenAddress}\"";
             StateDirectory = "tvix-daemon-${cache.name}";
             RuntimeDirectory = "tvix-daemon-${cache.name}";
-            DynamicUser = true;
+            User = "tvix-castore";
             Group = "tvix-castore";
             ReadWritePaths = cfg.castoreDir;
           } // systemdHardening;
@@ -158,7 +159,7 @@ in
           serviceConfig = {
             UMask = "007";
             ExecStart = "${pkgs.tvix}/bin/nar-bridge --otlp=false --listen-address=\"${cache.narBridgeListenAddress}\"";
-            DynamicUser = true;
+            User = "tvix-castore";
             Group = "tvix-castore";
             RuntimeDirectory = "narbridge-${cache.name}";
             ReadWritePaths = cfg.castoreDir;