Use of uninitialized value in String

From: Anish Kumar K. (anish_at_vitalect-india.com)
Date: 12/28/04 

To: "beginners perl" <beginners@perl.org>
Date: Tue, 28 Dec 2004 16:37:02 +0530

I am getting this error not sure why
   Use of uninitialized value in string eq at line :6

1: $rows=$dbh->prepare("select browsername from course where clientname ='DEMO");
2: $rows->execute();
3: while(@row=$rows->fetchrow_array())
4: {
5: $browserName=shift(@row);
6: if ($browserName eq "")
7: {
8: ++$browserHash{"EMPTY_BROWSER"};
9: }
10: else
11: {
12: ++$browserHash{$browserName};
13: }
14: }

Why this is happening ...The hash is like this

%browserHash=(
        "IE"=>0,
        "NETSCAPE"=>0,
        "FIREFOX"=>0,
        "OTHER"=>0,
        "EMPTY_BROWSER"=>0
);