Error in (function (classes, fdef, mtable) - topic-modeling

Question

I have run this topic modeling script two months ago SUCCESSFULLY, but it suddenly gives me an error message (in the last three lines).
post <- posterior(TM1, newdata = dtm[-c(1:20),]) #this script gives me an error message.
perplex <- perplexity(TM1, newdata = dtm[-c(1:20),]) #this script does not give me an error message.
Can anybody help me what is going on here? Please~~
=====================
library("tm")
library("slam")
library("topicmodels")
library("SnowballC")
corpus <- Corpus(DirSource(directory="/Users/loni/Documents/TextMining/test", encoding="UTF-8"))
dtm <- DocumentTermMatrix(corpus, control=list(stemming=TRUE, stopwords=TRUE, removePunctuation=FALSE))
term_tfidf <- tapply(dtm$v/row_sums(dtm)[dtm$i], dtm$j, mean) * log2(nDocs(dtm)/col_sums(dtm>0))
dim(dtm)
[1] 26 919
dtm <- dtm[, term_tfidf >= .06] # petition corpus
dtm <- dtm[row_sums(dtm) > 0,]
dim(dtm)
[1] 26 499
k<-5
SEED <- 2
TM <- list(VEM = LDA(dtm, k = k, control = list(seed = SEED)))
TM1 <- list(VEM = LDA(dtm[c(1:20),], k = k, control = list(seed = SEED))) #validation
Topic <- topics(TM[["VEM"]],1)
Terms <- terms(TM[["VEM"]], 8)
Terms[, 1:5]
post <- posterior(TM1, newdata = dtm[-c(1:20),])
Error in (function (classes, fdef, mtable) :
unable to find an inherited method for function ‘posterior’ for signature ‘"list", "DocumentTermMatrix"’

Answer 1

It could be because of wrong indexing of list. Try [[]] or [] on TM1

Answer 2

I had the same error today and found that the issue was because I had other packages loaded that conflicted. The easiest fix was to create a new session with a clear workspace, and rerun the script.
This answer to a similar question clued me in:
Unable to find an inherited method for function ‘select’ for signature ‘"data.frame"’