Contribute Page: Empty credit card field yields error

If the credit card field on a contribute page is left blank and the form is submitted, the following error displays:

Warning: str_repeat() [function.str-repeat]: Second argument has to be greater than or equal to 0. in /home/fiasiorg/public_html/joomla15/administrator/components/com_civicrm/civicrm/CRM/Utils/System.php on line 459

The following changes to the function at that line corrects the problem, but I'm unsure at this point if there are other implications to the fix:

static function mungeCreditCard( $number, $keep = 4 ) {
if ( $number !== '' )

{ $replace = str_repeat( '*' , strlen( $number ) - $keep ); }

return substr_replace( $number, $replace, 0, -$keep );
}