diff --git a/nixos/modules/installer/tools/nixos-rebuild.sh b/nixos/modules/installer/tools/nixos-rebuild.sh
index d7b749573fa..be37e61151a 100644
--- a/nixos/modules/installer/tools/nixos-rebuild.sh
+++ b/nixos/modules/installer/tools/nixos-rebuild.sh
@@ -97,6 +97,16 @@ if [ -n "$upgrade" -a -z "$_NIXOS_REBUILD_REEXEC" ]; then
     nix-channel --update nixos
 fi
 
+# Make sure that we use the Nix package we depend on, not something
+# else from the PATH for nix-{env,instantiate,build}.  This is
+# important, because NixOS defaults the architecture of the rebuilt
+# system to the architecture of the nix-* binaries used.  So if on an
+# amd64 system the user has an i686 Nix package in her PATH, then we
+# would silently downgrade the whole system to be i686 NixOS on the
+# next reboot.
+if [ -z "$_NIXOS_REBUILD_REEXEC" ]; then
+    export PATH=@nix@/bin:$PATH
+fi
 
 # Re-execute nixos-rebuild from the Nixpkgs tree.
 if [ -z "$_NIXOS_REBUILD_REEXEC" -a -n "$canRun" ]; then
diff --git a/nixos/modules/installer/tools/tools.nix b/nixos/modules/installer/tools/tools.nix
index 5ebf05e340f..f7fac75eb06 100644
--- a/nixos/modules/installer/tools/tools.nix
+++ b/nixos/modules/installer/tools/tools.nix
@@ -32,6 +32,7 @@ let
   nixos-rebuild = makeProg {
     name = "nixos-rebuild";
     src = ./nixos-rebuild.sh;
+    nix = config.nix.package;
   };
 
   nixos-generate-config = makeProg {