Undefined Index

Tja! Får denna error:

Notice: Undefined index: uid in B:\wamp\www\cb\login.php on line 3

när jag öppnar sidan med denna kod:

<?php
include "./global.php";
if($_SESSION['uid']){
$sql = "SELECT * FROM `users` WHERE `id`='".$_SESSION['uid']."'";
$res = mysql_query($sql) or die(mysql_error());

if(mysql_num_rows($res) == '0'){
session_destroy();
echo "<form action=\"index.php\" method=\"post\">";
echo "<table colspan=\"0\" cellspacing=\"3\" cellpadding=\"3\">";
echo "<tr><td>Username:</td><td><input type=\"text\" name=\"username\"></td></tr>";
echo "<tr><td>Password:</td><td><input type=\"password\" name=\"password\"></td></tr>";
echo "<tr><td colspan=\"2\" align=\"right\"><input type=\"submit\" name=\"submit\" value=\"Login\"></td></tr>";
}else {
echo "Welcome back, $row[username]!";
}
}else {
echo "<form action=\"index.php\" method=\"post\">";
echo "<table colspan=\"0\" cellspacing=\"3\" cellpadding=\"3\">";
echo "<tr><td>Username:</td><td><input type=\"text\" name=\"username\"></td></tr>";
echo "<tr><td>Password:</td><td><input type=\"password\" name=\"password\"></td></tr>";
echo "<tr><td colspan=\"2\" align=\"right\"><input type=\"submit\" name=\"submit\" value=\"Login\"></td></tr>";
}

?>

Vad är det för fel? Jag kan inte hitta det!