带有tm软件包的R工具

Question

运行此R代码时出现此问题

x = read.csv("data2.csv", header = TRUE)
library(tm)
a <-Corpus(DataframeSource(x), readerControl = list(language="english"))
a <- tm_map(a, as.PlainTextDocument)

<simpleError in UseMethod("as.PlainTextDocument", x): 
   no applicable method for 'as.PlainTextDocument' applied 
   to an object of class "c('PlainTextDocument', 'TextDocument', 'character')">

我安装了SnowballC个软件包，并在ubuntu下运行R. data2.csv是一个包含字符串行的文件。