R: function with optional arguments in data.table

I am trying to make a function with optional arguments. As of now the function works if all arguments are specified, but it breaks down when I leave one of the optional arguments out.

Here are the test data.tables:

library("data.table")
set.seed(2018)

ref_test_dt = data.table(FV = c(50000,sample((100:10000), 40, replace = T)),
                 msd = c(sample(1:100,41,replace = T)),
                 Product = c(rep("Cards",5),rep("Loans",36)))

target_test_dt = data.table(FV = c(70000,65800,sample((100:10000), 20, replace = T)), 
                            msd = c(sample(1:100,22,replace = T)))

my function:

scatter_test_f = function(x, y, target = NULL, ref = NULL){
       x = deparse(substitute(x))
       y = deparse(substitute(y))

if (!is.null(ref)){
  ref_compress = ref[, .(ref[[x]], ref[[y]])]
  setnames(ref_compress,c("V1","V2"),c(x, y))
  ref_add = ref_compress[, c("countlines", "Dataset") := .(1, "Reference")]
  ref_add[]
}

else if (!is.null(target)){
  target_compress = target[, .(target[[x]], target[[y]])]
  setnames(target_compress,c("V1","V2"),c(x, y))
  target_add = target_compress[,c("countlines", "Dataset") := .(1, "Target")]
  target_add[]
}

else
  target_compress = target[, .(target[[x]], target[[y]])]
  setnames(target_compress,c("V1","V2"),c(x, y))
  target_add = target_compress[,c("countlines", "Dataset") := .(1, "Target")]

  ref_compress = ref[, .(ref[[x]], ref[[y]])]
  setnames(ref_compress,c("V1","V2"),c(x, y))
  ref_add = ref_compress[, c("countlines", "Dataset") := .(1, "Reference")]

  ggData = rbind(target_add, ref_add)
  ggData[]
}

scatter_test_f(FV,msd, target_test_dt, ref_test_dt)

When only one optional argument is specified I get errors:

First error message gives me: Error in setnames(target_compress, c("V1", "V2"), c(x, y)) : Items of 'old' not found in column names: V1,V2

If I comment out the setnames then I will get the following message: Error in .(ref[[x]], ref[[y]]) : could not find function "."

Oddly enough (to me), when I have both optional argument specified, there are no errors. Anyone know why?