XPath concat in Query XML Action

Hello,

Im trying to concat two attributes but unfortunately i receive always this error message

The execution returned an unexpected error.

Expression must evaluate to a node-set

<RepeaterData>

<Version />

<Items>

<Item>

<Name type="System.String">Gullo</Name>

<Nationalitaet type="System.String">Italy</Nationalitaet>

<Vorname type="System.String">Domenico</Vorname>

<Typ type="System.String">Ehemann</Typ>

<Geburtsdatum type="System.DateTime"></Geburtsdatum>

<Groesse type="System.String"></Groesse>

<Heimatort type="System.String">Italy</Heimatort>

</Item>

</Items>

</RepeaterData>

My XPath is

concat(/RepeaterData/Items/Item/Vorname, ' ', /RepeaterData/Items/Item/Name)

If i run only /RepeaterData/Items/Item/Vorname it will give me back the FirstName, but the concat doesn't work.

Can somebody tell me whats wrong here?

thanks and regards

domenico