Session Cache PHP Warning

[aashidham]

Warning: session_start(): Cannot send session cache limiter - headers already sent (output started at /home/aashidham/webapps/decorya/canvas/header.php:20) in /home/aashidham/webapps/decorya/canvas/header.php on line 21

Appears when I load http://decorya.com/canvas/index.php?canvas=1#blah on Firefox