diff --git a/upstart-jobs/xserver.nix b/upstart-jobs/xserver.nix
index 0e204282c140..51004d457026 100644
--- a/upstart-jobs/xserver.nix
+++ b/upstart-jobs/xserver.nix
@@ -586,7 +586,7 @@ let
     ''
 
     else abort ("unknown window manager " + windowManager)}
-
+    WMpid=$!
 
     ### Show a background image.
     # (but not if we're starting a full desktop environment that does it for us)