在R Caret程序包的预测函数中找不到对象“ nn”

Question

当我在构建四种算法的集成模型之后，在下面的代码中运行预测函数，然后尝试使用该集成模型对新数据进行预测时，出现以下错误：未找到对象“ nn”

     models2 <- caretList(
     log_sales~., data=train.data.final,
     trControl=trnCtrl,  tuneList=list(
     rf=caretModelSpec(method="rf", tuneGrid=expand.grid(.mtry=c(1:8)) ),  
     nn=caretModelSpec(method="nnet", tuneGrid=expand.grid(.decay = 
     c(0,0.001,0.01, 0.1, 0.5), .size = c(5:10)), preProcess=c("center", 
     "scale"),linout=TRUE, trace=FALSE, maxit = 1000),
     gbm=caretModelSpec(method="gbm", 
     tuneGrid=expand.grid(interaction.depth = seq(1, 7, by = 2), n.trees = 
     seq(100, 1000, by = 50),shrinkage = c(0.01,0.1), n.minobsinnode = 10) 
     ),
     xgb=caretModelSpec(method="xgbTree", tuneGrid=expand.grid(nrounds = 
     1000,eta = c(0.01,0.05, 0.07, 0.1), max_depth = c(4:14), gamma=0, 
     colsample_bytree = c(0.75), subsample = c(0.50), min_child_weight =0 ) )

  )
  )

  stack.glm <- caretStack(models2, method="glm",  trControl=trnCtrl)
  stack.pred <- predict(stack.glm, newdata = testing)

eval（predvars，data，env）中的错误：找不到对象'nn'