Given the following:

library(biomaRt)
mart <- useMart("ensembl")
list <- listDatasets(m)
dataset.name <- list[ grep("sapiens",list$dataset, ignore.case=TRUE), 'dataset' ]

list clearly shows a data.frame containing character strings, is.character( dataset.name) is TRUE, dataset.name == "hsapiens_gene_ensembl" is also TRUE, yet the following call:

set <- useDataset(dataset.name, mart=mart)

gives

Error in (function (cl, name, valueClass)  :
  assignment of an object of class "AsIs" is not valid for @'dataset' in an object of class "Mart"; is(value, "character") is not TRUE

but

set <- useDataset("hsapiens_gene_ensembl", mart=mart)

works fine. I consider this a bug, and (to the more casual end user), a very intractible one. If data.set is, for all intents and purposes, a character string, then useDataset really should (try to) coerce this argument to a chr before crashing. Or should I report this elsewehere? Cheers,

Philip

Oh and the details:

> sessionInfo()
R version 3.3.0 (2016-05-03)
Platform: x86_64-pc-linux-gnu (64-bit)
Running under: CentOS Linux 7 (Core)

locale:
[1] C

attached base packages:
[1] stats     datasets  graphics  grDevices utils     methods   base     

other attached packages:
[1] biomaRt_2.30.0

loaded via a namespace (and not attached):
 [1] IRanges_2.6.1        parallel_3.3.0       DBI_0.5-1           
 [4] RCurl_1.95-4.8       memoise_1.0.0        Rcpp_0.12.8         
 [7] Biobase_2.34.0       AnnotationDbi_1.36.0 RSQLite_1.1         
[10] S4Vectors_0.10.3     BiocGenerics_0.20.0  digest_0.6.10       
[13] stats4_3.3.0         bitops_1.0-6         XML_3.98-1.5        
>

Thanks for reporting this.  I've patched it in the developmental version of the package.  At the same time I've also prevented multiple datasets being set in a particular mart object, which could cause problems when the user starts creating queries.  These changes should be available in biomaRt version 2.31.2