问文本挖掘中出错:二元运算符的非数字参数

EN

library(tm)
library(stringr)
mydata = data.frame(id=c(1,2,3), 
          text=c("This is text mining exercise;Meta-analysis in envtal science",
                 "Text analysis is bit confusing;This is how the analysis works",
                 "Hint on this text analysis?;Highly appreciated!")) 
multiwords<-c("Text","analysis","bit confusing")
txt = frq = nenv = prnt = list() 
for(i in 1:length(mydata$id)){ 
    nref[i] = lengths(strsplit(as.character(mydata$text[i]),";"))
    txt[i] = str_extract_all(mydata$text[i],str_c(multiwords, collapse="|"))
    frq[i] = lapply(txt[i],table)
    nenv[i] = sum(unlist(frq[i]))
    prnt[i] = nenv[i]/nref[i]*100
}