The query to the BioMart webservice returned an invalid result: biomaRt expected a character string

[Lorena Buono]

Hello, I have a problem using the function getBM in BioMart. I am using this command: 

library(biomaRt)

ensembl = useMart("ensembl",dataset="drerio_gene_ensembl")

Result<- getBM(attributes=c("ensembl_gene_id","external_gene_name"), filters = "go", values = 'GO:0005856', mart =ensembl)

But sometimes R return me this error:

Error in getBM(attributes = c("ensembl_gene_id", "external_gene_name"),  : 

  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

The weird fact is that this is not happening everytime, sometimes it does, sometimes it doesn't. I am also trying with different GO numbers and it looks like it doesn't depend on it. For example, I try the first time with the GO x it doesn't work but if I re-try 30 seconds later with exactly the same command it works perfectly. I don't know how to figure it out this. Any suggestions?

Cheers,

Lorena