diff --git a/ChangeLog b/ChangeLog
index 7ad144ef7f..62622dea00 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+Thu Dec  2 09:30:58 2004  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* eval.c (thread_mark): mark thread group.  [ruby-dev:25020]
+
+	* eval.c (thgroup_add): check whether the argument is really a Thread.
+
 Thu Dec  2 07:57:16 2004  Yukihiro Matsumoto  <matz@ruby-lang.org>
 
 	* io.c (rb_io_ctl): [ruby-dev:25019]
diff --git a/eval.c b/eval.c
index 0462a1e7e3..7c1e62802f 100644
--- a/eval.c
+++ b/eval.c
@@ -9786,6 +9786,7 @@ thread_mark(th)
     rb_gc_mark(th->last_line);
     rb_gc_mark(th->last_match);
     rb_mark_tbl(th->locals);
+    rb_gc_mark(th->thgroup);
 
     /* mark data in copied stack */
     if (th == curr_thread) return;
@@ -12480,6 +12481,10 @@ thgroup_add(group, thread)
 
     rb_secure(4);
     th = rb_thread_check(thread);
+    if (!th->next || !th->prev) {
+	rb_raise(rb_eTypeError, "wrong argument type %s (expected Thread)",
+		 rb_obj_classname(thread));
+    }
 
     if (OBJ_FROZEN(group)) {
       rb_raise(rb_eThreadError, "can't move to the frozen thread group");