[BioC] biomaRt query

[Sebastien Gerega]

Hi,
there is probably a very simple answer to this question but why does the 
following code produce the output NULL?

ensembl = useMart("ensembl", dataset="hsapiens_gene_ensembl")
getBM(attributes = c("entrezgene", "hgnc_symbol"), filters = 
"entrezgene", values = "117581", mart = ensembl, na.value = NA)

The NCBI site has a hgnc symbol listed for the gene:
http://www.ncbi.nlm.nih.gov/sites/entrez?db=gene&cmd=Retrieve&dopt=Graphics&list_uids=117581

thanks,
Sebastien