From: Dave Thomas <dave@...>
Date: 2008-03-18T12:16:49+09:00
Subject: Bug in Thread.set_trace_func

@@ -2904,7 +2904,7 @@
      GetThreadPtr(obj, th);
      rb_thread_remove_event_hook(th, call_trace_func);

-    if (!NIL_P(trace)) {
+    if (NIL_P(trace)) {
  	return Qnil;
      }
      thread_add_trace_func(th, trace);