--- a/xpcom/threads/nsIRunnable.idl
+++ b/xpcom/threads/nsIRunnable.idl
@@ -32,13 +32,13 @@
  * and other provisions required by the GPL or the LGPL. If you do not delete
  * the provisions above, a recipient may use your version of this file under
  * the terms of any one of the MPL, the GPL or the LGPL.
  *
  * ***** END LICENSE BLOCK ***** */
 
 #include "nsISupports.idl"
 
-[scriptable, uuid(4a2abaf0-6886-11d3-9382-00104ba0fd40)]
+[scriptable, function, uuid(4a2abaf0-6886-11d3-9382-00104ba0fd40)]
 interface nsIRunnable : nsISupports
 {
     void run();
 };