diff --git a/ChangeLog b/ChangeLog
index e19fc5c5d0..d863f3eedd 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Thu Jun  5 20:24:15 2008  Yusuke Endoh  <mame@tsg.ne.jp>
+
+	* thread.c (thread_set_trace_func_m): fix check for proc argument.
+
 Thu Jun  5 20:17:29 2008  Yusuke Endoh  <mame@tsg.ne.jp>
 
 	* lib/rexml/document.rb (REXML::Document:write): leaky modification
diff --git a/thread.c b/thread.c
index ec168f2137..5d933e4aef 100644
--- a/thread.c
+++ b/thread.c
@@ -3032,7 +3032,7 @@ thread_set_trace_func_m(VALUE obj, VALUE trace)
     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);