Notice: Indirect modification of overloaded element of WP_Session has no effect

Hi,

I have a function that should update the session datas :

function save_session_data($key,$value){
    $session = WP_Session::get_instance();
    $session['pinim'][$key] = $value;
}

But this gives me

Notice: Indirect modification of overloaded element of WP_Session has no effect in…

Any ideas ?
Thanks !

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