apply（）中的错误：dim（X）必须具有正长度

Question

以下代码给出了错误。是什么导致错误，我该怎么做才能解决？

# Determine number of clusters
wss <- (nrow(donnees.test$y - esvr1.pred) - 1) * 
       sum(apply(donnees.test$y - esvr1.pred, 2, var))
for (i in 2:15) wss[i] <- sum(kmeans(donnees.test$y - esvr1.pred,
                                     centers=i)$withinss)
plot(1:15, wss, type="b", xlab="Number of Clusters",
     ylab="Within groups sum of squares") 
# K-Means Cluster Analysis
fit <- kmeans(donnees.test$y - esvr1.pred, 5) # 5 cluster solution
# get cluster means
aggregate(donnees.test$y - esvr1.pred, by=list(fit$cluster), FUN=mean)
# append cluster assignment
MainData <- data.frame(donnees.test$y - esvr1.pred, fit$cluster) 

错误：

Error in apply(donnees.test$y - esvr1.pred, 2, var) : 
  dim(X) must have a positive length