Undefined variable: $_SESSION

I'm getting E_NOTICE errors in a core CakePHP file when it tries to reference a never-set or unset session (cake/libs/cake_session.php line 372):

function read($name = null) {
    if (is_null($name)) {
        return $this->__returnSessionVars();
    }
    if (empty($name)) {
        return false;
    }
    $result = Set::classicExtract($_SESSION, $name);
}

I've done a search through my code (in the app/ directory) and I can't find references to $_SESSION or session_destroy. Am I missing anything?

This error shows up when I try to run any unit tests. Is this...normal? I've cleared out the cake/ directory and replaced it with another one (same version) just to make sure that I hadn't inadvertently modified anything in the core files, but I still get the same error. I'm not sure if this is just a flaw in the framework or something else.

EDIT

Here are the results of the test run on the command line:

Welcome to CakePHP v1.3.11 Console
---------------------------------------------------------------
App : app
Path: /var/www/program/app
---------------------------------------------------------------
CakePHP Test Shell
---------------------------------------------------------------
Running app case models/owners_equity
E_NOTICE: Undefined variable: _SESSION in /var/www/program/cake/libs/cake_session.php on line 372
E_NOTICE: Undefined variable: _SESSION in /var/www/program/cake/libs/cake_session.php on line 372
ERROR->Unexpected PHP error [Undefined variable: _SESSION] severity [E_NOTICE] in [/var/www/program/cake/libs/cake_session.php line 372]
    in testGenerateOwnerWithdrawals
    in BalanceTestCase
    in /var/www/program/app/tests/cases/models/owners_equity.test.php

ERROR->Unexpected PHP error [Undefined variable: _SESSION] severity [E_NOTICE] in [/var/www/program/cake/libs/cake_session.php line 372]
    in testGenerateOwnerWithdrawals
    in BalanceTestCase
    in /var/www/program/app/tests/cases/models/owners_equity.test.php

Turned out there was some extra code in the AppModel that was messing things up:

in beforeFind and afterFind:

App::Import("Session");
$session = new CakeSession();
$sim_id = $session->read("Simulation.id");

I don't know why, but that was what the problem was. Removing those lines fixed the issue I was having.