diff --git a/ChangeLog b/ChangeLog
index 6aad90da8f..da095aa1db 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Tue Jun 28 00:08:43 2011  Keiju Ishitsuka  <keiju@ishitsuka.com>
+
+	* lib/irb/workspace.rb: fix BUG#4793.
+
 Mon Jun 27 22:06:12 2011  KOSAKI Motohiro  <kosaki.motohiro@gmail.com>
 
 	* thread_pthread.c (consume_communication_pipe): don't use C99
diff --git a/lib/irb/workspace.rb b/lib/irb/workspace.rb
index 991df2e652..edc0d3b147 100644
--- a/lib/irb/workspace.rb
+++ b/lib/irb/workspace.rb
@@ -97,7 +97,7 @@ EOF
       when 3
 	return nil if bt =~ /irb\/.*\.rb/
 	return nil if bt =~ /irb\.rb/
-	bt.sub!(/:\s*in `irb_binding'/, '')
+	bt = bt.sub(/:\s*in `irb_binding'/, '')
       end
       bt
     end