session_regenerate_id(): Cannot regenerate session id - headers already sent

I have several unit tests for my controllers, and all throw an error with PHPUnit (v3.5.14) whenever they include a call to Yii::app()->user->login($identity);

I have searched through my code, and don't see any extraneous whitespace or echo/print statements, so I'm not sure what's causing the problem. If I comment out the login() call, everything works fine.

Has anyone else run into this?