diff --git a/nixos/modules/system/build.nix b/nixos/modules/system/build.nix
index 1539e5b53b0c..58dc3f0d4113 100644
--- a/nixos/modules/system/build.nix
+++ b/nixos/modules/system/build.nix
@@ -6,12 +6,15 @@ in
   options = {
 
     system.build = mkOption {
-      internal = true;
       default = {};
-      type = with types; lazyAttrsOf (uniq unspecified);
       description = ''
         Attribute set of derivations used to set up the system.
       '';
+      type = types.submoduleWith {
+        modules = [{
+          freeformType = with types; lazyAttrsOf (uniq unspecified);
+        }];
+      };
     };
 
   };