r - Error using DEXSeqDataSetFromHTSeq -

currently trying understand dexseq package. have design tsv file , 7 files contains counts. run following command

library("dexseq");  design=read.table("dexseq_design.tsv", header=true, row.names=1);  ecs = dexseqdatasetfromhtseq(countfiles=c("m0.txt", "m1.txt", "m2.txt", "m3.txt", "m4.txt", "m5.txt", "m6.txt", "m7.txt"), design=design, flattenedfile="genome.chr.gff"); 

the last command gives , error

error in class(sampledata) %in% c("data.frame") :  error in evaluating argument 'x' in selecting method function '%in%': error: argument "sampledata" missing, no default 

what error means , how fix it? while loading package dexseq there warning

warning message:   replacing previous import ‘ggplot2::position’ when loading ‘deseq2’