用R和json“尝试选择少于一个元素”

Question

我使用包smappR进行twitter收获和分析。在此循环中使用函数getTimeline（https://github.com/SMAPPNYU/smappR/blob/master/R/get-timeline.R）时：

#import excel-file with twitter usernames    
riksdagsledamoter <- read.xlsx("lista_riksdagsledamoter_twitter.xlsx", sheetIndex=1) 

# delete MPs with out twitter account  
twitterusers <- riksdagsledamoter[!(is.na(riksdagsledamoter$twitterusername)), ] 
row.names(twitterusers) <- seq(nrow(twitterusers)) 
names(twitterusers) <- c("twitterusername", "name", "parti") #convertering to vector
twitterusersvector <- as.vector(twitterusers$twitterusername)

    for(i in 1:length(twitterusersvector)){
      user <- twitterusersvector[i]
      # user <- twitterusers$twitterusername[i]
      # rov <- as.vector(user)
      getTimeline(screen_name = user,
                  filename = "pa324.json", # where tweets will be stored
                  n=3200, ## number of tweets to download (max is 3,200)
                  oauth_folder = "~/Dropbox/Privat/R/credentials",
                  sleep=10, )
      if ( file.info("pa324.json")$size > 120000000 ){
      tweetsToMongo(file.name="pa324.json", ns="xxxx.yyyy",
                    host="192.168.x.x", username="", password="")
      file.remove("pa324.json")
      }
    }

我收到此错误：

Error in json.data[[tweets]] : attempt to select less than one element

我用Google搜索并尝试过：

row.names(twitterusers) <- seq(nrow(twitterusers)) 

没有任何帮助。可能有什么不对？