Conflict with Woocommerce plugin

Hello,

Here is my problem. When I make a backup the following message appears :

Warning: session_start() [function.session-start]: Cannot send session cache limiter – headers already sent (output started at /*****/*****/www/wp-includes/functions.php:2504) in /*****/*****/www/wp-content/plugins/woocommerce/woocommerce.php on line 138

But the backup seems to end successfully : a new extentionless file is created on my drive account.

Is that normal? Do I have to worry about this issue?

Thanks for support.

http://wordpress.org/extend/plugins/backup/