Session value not working when FPC cache enabled

Session value not working in blocks when FPC cache enabled.How to handle the session value in cache enabled blocks.I have added the customer session id as below.How to get the customer id from cache in a block.

public function getCacheKeyInfo() {    $info = parent::getCacheKeyInfo();    $info['current_product_id'] = Mage::registry('current_product')->getId();    $info['customer_id'] = Mage::getSingleton('customer/session')->getCustomerId();    return $info;}