diff --git a/nixos-modules/genode-core.nix b/nixos-modules/genode-core.nix
index 0a4a5d6..6447e2c 100644
--- a/nixos-modules/genode-core.nix
+++ b/nixos-modules/genode-core.nix
@@ -81,6 +81,7 @@ in {
 
       romModules = mkOption {
         type = types.attrsOf types.path;
+        default = { };
         description = "Attr set of initial ROM modules";
       };