[fw-general] Action Helper FlashMessenger problem

[Kamil]

i tried to use FlashMessenger
$this->_flashMessenger = $this->getHelper('FlashMessenger');
$this->_flashMessenger->addMessage('some text');
i get error
<b>Notice</b>: Indirect modification of overloaded property 
Zend_Session_Namespace::$default has no effect in 
<b>/library/Zend/Controller/Action/Helper/FlashMessenger.php</b> on line 
<b>133</b><br />
or   when i am using $this->_flashMessenger->setNamespace('back');
the same error
<b>Notice</b >: Indirect modification of overloaded property 
Zend_Session_Namespace::$back
what i am doing wrong  please help