diff --git a/srfi/18.sld b/srfi/18.sld
index 02499e6c..a7ee9a3b 100644
--- a/srfi/18.sld
+++ b/srfi/18.sld
@@ -74,6 +74,7 @@
         ;; - internal
         ;; - end of thread cont (or #f for default)
         ;; - end-result - Result of thread that terminates successfully
+        ;; - internal thread context at termination, e.g. parameterised objects
         (vector 
           'cyc-thread-obj 
           thunk 
@@ -82,6 +83,7 @@
           #f 
           #f
           #f
+          #f
           #f)))
 
     (define (thread-name t) (vector-ref t 3))