如果我执行以下代码，我没有问题：

require(foreign)
require(nnet)
require(ggplot2)
require(reshape2)

ml <- read.dta("https://stats.idre.ucla.edu/stat/data/hsbdemo.dta")
ml$prog2 <- relevel(ml$prog, ref = "academic")
test <- multinom(prog2 ~ ses + write, data = ml)
predict(test, newdata = dses, "probs")

但如果我尝试：

require(caret)
ml <- read.dta("http://www.ats.ucla.edu/stat/data/hsbdemo.dta")
ml$prog2 <- relevel(ml$prog, ref = "academic")
test <- train(prog2 ~ ses + write,method="multinom" ,data = ml)
predict(test$finalModel, newdata = dses, "probs")

它返回Error in eval(expr, envir, enclos) : object 'sesmiddle' not found，为什么？