diff --git a/src/ChangeLog b/src/ChangeLog
index f5cc4463bea..49936d11a5f 100644
--- a/src/ChangeLog
+++ b/src/ChangeLog
@@ -1,5 +1,8 @@
 2009-04-09  Kenichi Handa  <handa@m17n.org>
 
+	* charset.c (map_charset_chars): For a charset of `superset'
+	method, fix calculation of code range.
+
 	* font.c (font_put_extra): If VAL is nil, delete the slot for PROP
 	from the list of extra properties.
 	(font_clear_prop): Be sure to delete `:name' font property.
diff --git a/src/charset.c b/src/charset.c
index 54602ebe21d..492849f6868 100644
--- a/src/charset.c
+++ b/src/charset.c
@@ -809,8 +809,8 @@ map_charset_chars (c_function, function, arg,
 
 	  charset = CHARSET_FROM_ID (XFASTINT (XCAR (XCAR (parents))));
 	  offset = XINT (XCDR (XCAR (parents)));
-	  this_from = from - offset;
-	  this_to = to - offset;
+	  this_from = from > offset ? from - offset : 0;
+	  this_to = to > offset ? to - offset : 0;
 	  if (this_from < CHARSET_MIN_CODE (charset))
 	    this_from = CHARSET_MIN_CODE (charset);
 	  if (this_to > CHARSET_MAX_CODE (charset))