I am trying to convert from ensembl ID to gene symbol. Here is the chunk of code I'm using:

ensembl = useMart( "ensembl", dataset = "hsapiens_gene_ensembl")
genemap <- getBM( attributes = c("ensembl_gene_id", "entrezgene", "hgnc_symbol"),
                  filters = "ensembl_gene_id",
                  values = dat$ensembl,
                  mart = ensembl )
idx <- match( dat$ensembl, genemap$ensembl_gene_id )
dat$entrez <- genemap$entrezgene[idx]
dat$hgnc_symbol <- genemap$hgnc_symbol[idx]
dat <- subset(dat, select = -c(ensembl, entrez))

I get this error when I run it:

The query to the BioMart webservice returned an invalid result: biomaRt expected a character string of length 1. 
Please report this on the support site at http://support.bioconductor.org

Anyone able to help? Thanks.

Use useEnsembl, rather than useMart, and choose the closest Ensembl mirror to you. But do choose a mirror, as the main site seems to be the (consistent) problem here.