Accessing data in bind

Hi,

I want to bind some data to a field in a grid. The data is in another instance and it is picked by matching a given ID. I do this:

 <xforms:bind name="descricao_informacao" id="descricao_informacao-bind"
                                     ref="instance('dados-externos')/dado_externo[@id=codigo_informacao']/ecfdesc"
                                     type="xforms:string"> 

But it doesn't work. But if I put it hard coded like this:

 <xforms:bind name="descricao_informacao" id="descricao_informacao-bind"
                                     ref="instance('dados-externos')/dado_externo[@id=444]/ecfdesc"
                                     type="xforms:string"> 

Then it works, how can I get this to work referencing the actual "codigo_informacao" element?

Henrique Barros