diff --git a/ChangeLog b/ChangeLog
index 035f25899d..1952918a3a 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Sun Aug 24 04:23:19 2008  NARUSE, Yui  <naruse@ruby-lang.org>
+
+	* encoding.c (enc_compatible_p): raise TypeError when argument is Encoding.
+
 Sat Aug 23 23:25:47 2008  NARUSE, Yui  <naruse@ruby-lang.org>
 
 	* lib/cgi.rb (CGI#out): remove auto encoding conversion and set
diff --git a/encoding.c b/encoding.c
index 9002a1f17f..5e8412a026 100644
--- a/encoding.c
+++ b/encoding.c
@@ -876,6 +876,8 @@ enc_compatible_p(VALUE klass, VALUE str1, VALUE str2)
 {
     rb_encoding *enc;
 
+    if (enc_check_encoding(str1) > 0 || enc_check_encoding(str2) > 0)
+       rb_raise(rb_eTypeError, "wrong argument type Encoding (expected String)");
     if (!enc_capable(str1)) return Qnil;
     if (!enc_capable(str2)) return Qnil;
     enc = rb_enc_compatible(str1, str2);