I install isLinked mod. when i try to add a site without URL, it keeps saying - isLinked (Can not be left blank)

I did some changes as the following :
links.def
Û_def = (
ID => [0, 'numer', 5, 8, 1, '', ''],
Title => [1, 'alpha', 40, 75, 1, '', ''],
URL => [2, 'alpha', 40, 75, 0, 'http://', '^http|news|mailto|ftp'],
Date => [3, 'date', 15, 15, 1, \&get_date, ''],
Category => [4, 'alpha', 0, 150, 1, '', ''],
Description => [5, 'alpha', '40x3', 500, 0, '', ''],
'Contact Name' => [6, 'alpha', 40, 75, 1, '', ''],
'Contact Email' => [7, 'alpha', 40, 75, 1, '', '.+@.+\..+'],
Hits => [8, 'numer', 10, 10, 1, '0', '\d+'],
isNew => [9, 'alpha', 0, 5, 0, 'No', ''],
isPopular => [10, 'alpha', 0, 5, 0, 'No', ''],
Rating => [11, 'numer', 10, 10, 1, 0, '^[\d\.]+$'],
Votes => [12, 'numer', 10, 10, 1, 0, '^\d+$'],
ReceiveMail => [13, 'alpha', 10, 10, 1, 'Yes', 'No|Yes'],
isLinked => [14, 'alpha', 0, 5, 1, 'No', 'No|Yes']

);

# where the first field equals 0.
$db_category = 4; $db_modified = 3; $db_url = 2;
$db_hits = 8; $db_isnew = 9; $db_ispop = 10;
$db_contact_name = 6; $db_contact_email = 7; $db_title = 1;
$db_votes = 12; $db_rating = 11; $db_mail = 13;
$db_islinked = 14;

Û_select_fields = (
isNew => 'Yes,No',
isPopular => 'Yes,No',
ReceiveMail => 'Yes,No',
isLinked => 'Yes,No'
);

site_html_templates.pl
# Set new and pop to either 1 or 0 for templates.
($rec{'isNew'} eq 'Yes') ? ($rec{'isNew'} = 1) : (delete $rec{'isNew'});
($rec{'isPopular'} eq 'Yes') ? ($rec{'isPopular'} = 1) : (delete $rec{'isPopular'});
($rec{'isLinked'} eq 'Yes') ? ($rec{'isLinked'} = 1) : (delete $rec{'isLinked'});

link.html
<%if isLinked%>
<a href="<%URL%>" target="_Blank"><%Title%></a>
<%endif%>
<%ifnot isLinked%><%Title%><%endif%>

Please help

Thanks