diff --git a/harbour/ChangeLog b/harbour/ChangeLog
index 0492b77467..267d035873 100644
--- a/harbour/ChangeLog
+++ b/harbour/ChangeLog
@@ -8,6 +8,12 @@
    2002-12-01 23:12 UTC+0100 Foo Bar <foo.bar@foobar.org>
 */
 
+2003-05-06 15:35 UTC+0300 Alexander Kresin <alex@belacy.belgorod.su>
+  * source/rtl/is.c
+    ! Bug fixed: ISUPPER, ISLOWER and ISALPHA with chr(0) argument
+      returned TRUE.
+      Patch provided by Przemyslaw Czerpak
+
 2003-05-06 12:55 UTC+0200 Przemyslaw Czerpak <druzus@polbox.com>
   * source/rtl/set.c
     ! Fixed very serious bug in set(_SET_ALTFILE, "") and
diff --git a/harbour/source/rtl/is.c b/harbour/source/rtl/is.c
index a2332782c3..33e684cc05 100644
--- a/harbour/source/rtl/is.c
+++ b/harbour/source/rtl/is.c
@@ -67,7 +67,7 @@ HB_FUNC( ISALPHA )
    {
       if( isalpha( ( unsigned char ) * szString ) )
          hb_retl( TRUE );
-      else if( s_cdpage->nChars && 
+      else if( s_cdpage->nChars && szString[0] && 
            ( strchr( s_cdpage->CharsUpper,* szString ) || strchr( s_cdpage->CharsLower,* szString ) ) )
          hb_retl( TRUE );
       else
@@ -99,7 +99,7 @@ HB_FUNC( ISUPPER )
    {
       if( isupper( ( unsigned char ) * szString ) )
          hb_retl( TRUE );
-      else if( s_cdpage->nChars && strchr( s_cdpage->CharsUpper,* szString ) )
+      else if( s_cdpage->nChars && szString[0] && strchr( s_cdpage->CharsUpper,* szString ) )
          hb_retl( TRUE );
       else
          hb_retl( FALSE );
@@ -118,7 +118,7 @@ HB_FUNC( ISLOWER )
    {
       if( islower( ( unsigned char ) * szString ) )
          hb_retl( TRUE );
-      else if( s_cdpage->nChars && strchr( s_cdpage->CharsLower,* szString ) )
+      else if( s_cdpage->nChars && szString[0] && strchr( s_cdpage->CharsLower,* szString ) )
          hb_retl( TRUE );
       else
          hb_retl( FALSE );