Error in is.single.string(object) : argument "object" is missing, with no default

Question

I want to parse the AAChange.refGene column and then use biomaRt R package to extract information. My code is raising Error in is.single.string(object) : argument "object" is missing, with no default even though the getSequence function is meant to accept multiple arguments.

library(tidyr)

variant_calls = read.delim("variant_calls.txt")
info = tidyr::separate(variant_calls["AAChange.refGene"], AAChange.refGene, c("Refseq ID", "cDNA level change", "Protein level change"), ":")
df = cbind(variant_calls["Gene.refGene"],info) 

library(biomaRt)
ensembl <- useMart(biomart="ENSEMBL_MART_ENSEMBL", dataset="hsapiens_gene_ensembl", host="https://grch37.ensembl.org", path="/biomart/martservice")

pep <- vector()
for(i in 1:length(df$`Refseq ID`)){
  temp <- getSequence(id=df$`Refseq ID`[i],type='refseq_mrna',seqType='peptide', mart=ensembl)
  temp <- sapply(temp$peptide, nchar)
  temp <- sort(temp, decreasing = TRUE)
  temp <- names(temp[1])
  pep[i] <- temp
}
df$Sequence <- pep

Traceback:

Error in is.single.string(object) : 
  argument "object" is missing, with no default