Storing multidimensional array in session variable

Hi, 
I want  to store multidimensional array in session variable, 

In my method I store 3 dimensional array $this->session->set_userdata('shopping_cart', $cart); in this way,
 and after this line I printed it print_r($this->session->userdata('shopping_cart')); and this is retrieving the data I need, but after the method ends shopping_cart is erased from session somehow. I tried to put 1 dimesional array in session variable, it worked with no problems, but it is not working with multidimensional arrays.