custom session variables

Im trying to use session variables for one of my projects ,
but they dont seem to hold, on the next page load the session variable is not there, all other variables of amember pro are there,

Code:

if(empty($_SESSION)) session_start();

if(!isset($_SESSION['var1']))
{
 $_SESSION['var1'] = 100;
 echo "set";
}
else
{
 echo "get: ".$_SESSION['var1'];
}

it always ends up in the first loop.

please advise.
M.

Start page with just - session_start();