diff --git a/pkgs/os-specific/linux/systemd/default.nix b/pkgs/os-specific/linux/systemd/default.nix
index dbf877ebaeb..a6889e3e440 100644
--- a/pkgs/os-specific/linux/systemd/default.nix
+++ b/pkgs/os-specific/linux/systemd/default.nix
@@ -70,6 +70,14 @@ stdenv.mkDerivation rec {
 
   enableParallelBuilding = true;
   
+  # The interface version prevents NixOS from switching to an
+  # incompatible systemd at runtime.  (Switching across reboots is
+  # fine, of course.)  It should be increased whenever systemd changes
+  # in a backwards-incompatible way.  If the interface version of two
+  # systemd builds is the same, then we can switch between them at
+  # runtime; otherwise we can't and we need to reboot.
+  passthru.interfaceVersion = 2;
+
   meta = {
     homepage = http://www.freedesktop.org/wiki/Software/systemd;
     description = "A system and service manager for Linux";