diff --git a/ChangeLog b/ChangeLog
index 9e2735e1e2..d46e0105fd 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Thu Dec 27 15:11:27 2007  Tanaka Akira  <akr@fsij.org>
+
+	* encoding.c (rb_enc_check): show encodings in error message.
+
 Thu Dec 27 15:02:52 2007  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* string.c (rb_str_casecmp): fixed using a wrong variable.
diff --git a/encoding.c b/encoding.c
index ea00deb9e2..a8c04dcebf 100644
--- a/encoding.c
+++ b/encoding.c
@@ -504,7 +504,9 @@ rb_enc_check(VALUE str1, VALUE str2)
 {
     rb_encoding *enc = rb_enc_compatible(str1, str2);
     if (!enc)
-	rb_raise(rb_eArgError, "character encodings differ");
+	rb_raise(rb_eArgError, "character encodings differ: %s and %s",
+            rb_enc_name(rb_enc_get(str1)),
+            rb_enc_name(rb_enc_get(str2)));
     return enc;
 }