diff --git a/runtime.c b/runtime.c
index 00c379b7..fa6129bd 100644
--- a/runtime.c
+++ b/runtime.c
@@ -6624,7 +6624,11 @@ void *Cyc_init_thread(object thread_and_thunk)
   thd->scm_thread_obj = car(thread_and_thunk);
   thd->gc_cont = cdr(thread_and_thunk);
   thd->gc_num_args = 1;
-  thd->gc_args[0] = &Cyc_91end_91thread_67_primitive;
+  if (t->num_elements >= 7 && t->elements[6] != boolean_f) {
+    thd->gc_args[0] = t->elements[6];
+  } else {
+    thd->gc_args[0] = &Cyc_91end_91thread_67_primitive;
+  }
   thd->thread_id = pthread_self();
 
   // Copy thread params from the calling thread
diff --git a/srfi/18.sld b/srfi/18.sld
index 9c126d25..d3ea0033 100644
--- a/srfi/18.sld
+++ b/srfi/18.sld
@@ -71,12 +71,14 @@
         ;; - name
         ;; - specific
         ;; - internal
+        ;; - end of thread cont (or #f for default)
         (vector 
           'cyc-thread-obj 
           thunk 
           (%alloc-thread-data)  ;; Internal data for new thread
           name-str 
           #f 
+          #f
           #f)))
 
     (define (thread-name t) (vector-ref t 3))