diff --git a/ChangeLog b/ChangeLog
index 73c25d4c0f..ee10726b23 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+Fri Apr  9 23:58:58 2010  Yusuke Endoh  <mame@tsg.ne.jp>
+
+	* ext/pty/lib/expect.rb: raise an error when argument is unexpected
+	  type.  based on a patch from Luiz Angelo Daros de Luca in
+	  [ruby-core:23464].
+
 Fri Apr  9 23:57:35 2010  Yusuke Endoh  <mame@tsg.ne.jp>
 
 	* ext/pty/lib/expect.rb: add rdoc.  based on a patch from Luiz Angelo
diff --git a/ext/pty/lib/expect.rb b/ext/pty/lib/expect.rb
index 50d2fbd363..52396d90bd 100644
--- a/ext/pty/lib/expect.rb
+++ b/ext/pty/lib/expect.rb
@@ -14,6 +14,8 @@ class IO
       e_pat = Regexp.new(Regexp.quote(pat))
     when Regexp
       e_pat = pat
+    else
+      raise TypeError, "unsupported pattern class: #{pattern.class}"
     end
     while true
       if !IO.select([self],nil,nil,timeout) or eof? then