R, Error: $ operator is invalid for atomic vectors in fastai

Question

Running CollabDataLoaders() in implementing fastai using R in the code below:

library(fastai)
library(zeallot)
library(magrittr)
library(data.table)

URLs_MOVIE_LENS_ML_100k()

c(user, item, title) %<-% list('userId', 'movieId', 'title')

ratings = fread('ml-100k/u.data', col.names = c(user,item,'rating','timestamp'))
movies = fread('ml-100k/u.item', col.names = c(item, 'title', 'date', 'N', 'url',
                                               paste('g',1:19,sep = '')))

rating_movie = ratings[movies[, .SD, .SDcols=c(item,title)], on = item]

dls = CollabDataLoaders_from_df(rating_movie, seed=42, valid_pct=0.1, bs=64, item_name=title, path='ml-100k')

Encountered an error in the last line:

dls = CollabDataLoaders_from_df(rating_movie, seed=42, valid_pct=0.1, bs=64, item_name=title, path='ml-100k')

Error: $ operator is invalid for atomic vectors

rating_movie is a data frame object without NA. Appreciate if you can help.