How Do I Fix the “No Encoding Supplied” Error?

I am facing difficulties after running the code and trying to export the dataset to a spreadsheet or txt.file. I am newbie to R, so maybe this question is trivial.

After running the following code:

eia_series <- function(api_key, series_id, start = NULL, end = NULL, num = NULL, tidy_data = "no", only_data = FALSE){
  # max 100 series
  # test if num is not null and either start or end is nut null. Not allowed
  # api_key test for character.
  # series_id test for character.
  # if start/end not null, then check if format matches series id date format
  # parse date and numerical data

  # parse url
  series_url <- httr::parse_url("")
  series_url$query$series_id <- paste(series_id, collapse = ";")
  series_url$query$api_key <- api_key
  series_url$query$start <- start
  series_url$query$end <- end
  series_url$query$num <- num

  # get data
  series_data <- httr::GET(url = series_url)
  series_data <- httr::content(series_data, as = "text")
  series_data <- jsonlite::fromJSON(series_data)

  # Move data from data.frame with nested list and NULL excisting
  series_data$data <- series_data$series$data
  series_data$series$data <- NULL

  # parse data
  series_data$data <- lapply(X = series_data$data, 
                         FUN = function(x) data.frame(date = x[, 1], 
                                                      value = as.numeric(x[, 2]), 
                                                      stringsAsFactors = FALSE))

  # add names to the list with data
  names(series_data$data) <- series_data$data

  # parse dates
  series_data$data <- eia_date_parse(series_list = series_data$data, format_character = series_data$series$f)

  # tidy up data
  if(tidy_data == "tidy_long"){

    series_data$data <- lapply(seq_along(series_data$data), 
                           function(x) {cbind(series_data$data[[x]], 
                                              series_time_frame = series_data$series$f[x],
                                              series_name = series_data$series$series_id[x], 
                                              stringsAsFactors = FALSE)})
series_data$data <- do.call(rbind, series_data$data)
  } 

  # only data
  if(only_data){
    series_data <- series_data$data
  }

  return(series_data)
}

After running the function

eia_series(api_key = "XXX",series_id = c("PET.MCRFPOK1.M", "PET.MCRFPOK2.M"))

I tried to "transfer" the data in order to export it but got the following error:

No encoding supplied: defaulting to UTF-8.

I don't understand why. Could you help me out?