diff --git a/nixos/lib/eval-config.nix b/nixos/lib/eval-config.nix
index 1e086271e523..058ab7280ccc 100644
--- a/nixos/lib/eval-config.nix
+++ b/nixos/lib/eval-config.nix
@@ -38,6 +38,8 @@ let pkgs_ = pkgs;
 in
 
 let
+  inherit (lib) optional;
+
   evalModulesMinimal = (import ./default.nix {
     inherit lib;
     # Implicit use of feature is noted in implementation.
@@ -47,15 +49,19 @@ let
   pkgsModule = rec {
     _file = ./eval-config.nix;
     key = _file;
-    config = {
-      # Explicit `nixpkgs.system` or `nixpkgs.localSystem` should override
-      # this.  Since the latter defaults to the former, the former should
-      # default to the argument. That way this new default could propagate all
-      # they way through, but has the last priority behind everything else.
-      nixpkgs.system = lib.mkIf (system != null) (lib.mkDefault system);
-
-      _module.args.pkgs = lib.mkIf (pkgs_ != null) (lib.mkForce pkgs_);
-    };
+    config = lib.mkMerge (
+      (optional (system != null) {
+        # Explicit `nixpkgs.system` or `nixpkgs.localSystem` should override
+        # this.  Since the latter defaults to the former, the former should
+        # default to the argument. That way this new default could propagate all
+        # they way through, but has the last priority behind everything else.
+        nixpkgs.system = lib.mkDefault system;
+      })
+      ++
+      (optional (pkgs_ != null) {
+        _module.args.pkgs = lib.mkForce pkgs_;
+      })
+    );
   };
 
   withWarnings = x: