[BioC] biomaRt UseMart

[Javier Díez]

Hy,
I'm working with R (Version 2.3.1) and Bioconductor(1.8) on Linux (Ubuntu).
I'm trying biomaRt, but after installing the package can't create a mart 
object; here it is the error returned. Can anybody help me?
Best regards.
Javi

 > mart <- useMart("ensembl")
Error in validObject(.Object) : invalid class "Mart" object: invalid 
object for slot "vschema" in class "Mart": got class "NULL", should be 
or extend class "character"