It should be nice to understand why always obtain NULL when using Rbowtie2 functions. The program its located in the library but it doesn´t work

library(Rbowtie2)
> td <- tempdir()
> refs <- dir(system.file(package="Rbowtie2", "extdata", "bt2","refs"),full=TRUE)
> (cmdout<-bowtie2_build(references=refs, 
+                        bt2Index=file.path(td, "lambda_virus"), "--threads 4 --quiet",
+                        overwrite=TRUE))
NULL