diff --git a/nixos/modules/system/activation/top-level.nix b/nixos/modules/system/activation/top-level.nix
index 1c588ff96918..8e53ec1ffab2 100644
--- a/nixos/modules/system/activation/top-level.nix
+++ b/nixos/modules/system/activation/top-level.nix
@@ -148,7 +148,7 @@ in
     system.build = mkOption {
       internal = true;
       default = {};
-      type = types.lazyAttrsOf types.unspecified;
+      type = with types; lazyAttrsOf (uniq unspecified);
       description = ''
         Attribute set of derivations used to setup the system.
       '';