求大佬救命
我按照书上给的代码运行随机森林总是出现
Error in model.frame.default(formula = a$quality ~ ., data = a.train,  :
  变数的长度不一样('fixed.acidity')
求解答

以下是代码：
rm(list = ls())
library(randomForest)
library(readxl)
library("rpart")
library("rpart.plot")
library("survival")
a<-read.csv("E1.1.csv")
a<-na.omit(a)
a$quality<-factor(a$quality,levels=c(3,4),labels=c("bad","good"))
train<-sample(nrow(a),0.7*nrow(a))
a.train<-a[train,]
a.validate<-a[-train,]
fit<-randomForest(a$quality~.,data = a.train,na.action = na.roughfix,importance=TRUE)