diff --git a/pkgs/top-level/impure.nix b/pkgs/top-level/impure.nix
index df462665dd16..dafa351c4e41 100644
--- a/pkgs/top-level/impure.nix
+++ b/pkgs/top-level/impure.nix
@@ -83,5 +83,6 @@ import ./. (builtins.removeAttrs args [ "system" "platform" ] // {
   inherit config overlays crossSystem;
   # Fallback: Assume we are building packages on the current (build, in GNU
   # Autotools parlance) system.
-  localSystem = { system = builtins.currentSystem; } // localSystem;
+  localSystem = (if args ? localSystem then {}
+                 else { system = builtins.currentSystem; }) // localSystem;
 })