Unable to request Managed service query

Hi, I'm tried to get all the client_id inside my MCC.

At the moment I'm using this code:

library(RAdwordsPlus)
library(RAdwords)

google_auth <- doAuth()

api_version <- "v201809"
customer_id <- "xxx-xxx-xxxx" #MCC ID
request <-  RAdwordsPlus::managed.customer.request(fields = c("Name", "CustomerId")) 

r <- RAdwordsPlus::get.service(request     = request,
                 cid         = customer_id,
                 auth        = google_auth,
                 api.version = api_version,
                 user.agent  = "r-adwordsplus-test",
                 verbose     = FALSE,
                 raw = FALSE,
                 partial.failure = FALSE)

But I got and error:

Warning message: In parser(response) : x is not a valid managed.customer

And an empty response. I've also try to insert as customer_id parameter one of my client_id but the error is the same. Not sure if it's possible in R but how can I get the list of all the my client_id listed inside my MCC?

Thanks

adviso / RAdwordsPlus

Unable to request Managed service query #2