Error with keepAlive tag

[acdias]

<a4j:keepAlive beanName="#{bancaController.banca}"/>

I tried to use this tag but I always get an error saiyng that bean must be literal. My Bean and ManagedBean implements Serializable.

<a4j:keepAlive beanName="#{bancaController.banca}"> Must be literal

Anyone knows what can be happen with this?
And where should I put this tag?