I get the following error when I run the ViSEAGO package

Error in .local(.Object, ...) : allGenes must be a factor with 2 levels

I have a list of differentially expressed genes (from a DE proteomics experiment) and wanted to use the ViSEAGO package to visualise the GO profile. If I understand correctly I need to supply my DE genes as a vector and a list of background genes as a separate vector. Where I am struggling is, what needs to be in these files? Obviously, gene ids need to be present and that's all I supplied. My code is as follows:

str(Downreg_gene_id_v4)

data.frame': 286 obs. of 1 variable:

$ Entrez_Gene_ID: int 2288 5898 7384 3028 11345 5704 7414 1659 84844 10291 ...

#set up gene list to be analysed and background list
write.table(Downreg_gene_id_v4, sep="\t", file="selection.txt")
selection <- read.table("selection.txt")
background=keys(org.Hs.eg.db, keytype ='ENTREZID')
write.table(background, sep="\t", file="background.txt")
background <- read.table("background.txt")
names(background)[1] <- "Entrez_Gene_ID"

from the package vingette

# connect to Entrezgene to get GO annotations
EntrezGene<-ViSEAGO::EntrezGene2GO()

# load GO annotations from EntrezGene
#id = "9606" = homo sapiens
myGENE2GO<-ViSEAGO::annotate(id="9606", EntrezGene)

# create topGOdata for BP
BP<-ViSEAGO::create_topGOdata(
geneSel=selection,
allGenes=background,
gene2GO=myGENE2GO, 
ont="BP",
nodeSize=5
)

It's at this point I get the error message

Error in .local(.Object, ...) : allGenes must be a factor with 2 levels

Any advice is gratefully recieved.