diff --git a/embedding/components/windowwatcher/public/nsIWindowWatcher.idl b/embedding/components/windowwatcher/public/nsIWindowWatcher.idl
index 8a648f6ad8f..3880fb66a3a 100644
--- a/embedding/components/windowwatcher/public/nsIWindowWatcher.idl
+++ b/embedding/components/windowwatcher/public/nsIWindowWatcher.idl
@@ -162,5 +162,6 @@ interface nsIWindowWatcher : nsISupports {
 
 };
 
+%{C++
 #define NS_WINDOWWATCHER_CONTRACTID "@mozilla.org/embedcomp/window-watcher;1"
 %}