我有此代码：
# Define training control
set.seed(123)
train.control <- trainControl(method = "cv", number = 10)
# Train the model
model <- train(is_nocnv ~., data = mydata, method = "lm", trControl = train.control)
# Summarize the results
print(model)
执行此代码时，我得到此错误：
Error in `contrasts<-`(`*tmp*`, value = contr.funs[1 + isOF[nn]]) :
contrasts can be applied only to factors with 2 or more levels
字段：is_nocnv是因子，该字段的值为'YES'，'NO'
str(mydata)
'data.frame': 8334 obs. of 7 variables:
$ chr : Factor w/ 1 level "chr1": 1 1 1 1 1 1 1 1 1 1 ...
$ start : int 3218610 154080441 154089408 61735 2069681 2074104 3135175 3137913 3214732 5901288 ...
$ stop : int 154074261 154081058 247813706 2061969 2071738 3130590 3136858 3212946 5900106 5902086 ...
$ strand : Factor w/ 1 level "*": 1 1 1 1 1 1 1 1 1 1 ...
$ num_probes : int 69643 3 59364 379 2 333 2 33 1943 3 ...
$ segment_mean: num -0.122 -13.462 -0.1 -0.326 -25.242 ...
$ is_nocnv : Factor w/ 2 levels "NO","YES": 2 2 2 1 1 1 1 1 1 1 ...
这是我的数据集（csv）的一小部分
"chr","start","stop","strand","num_probes","segment_mean","is_nocnv"
chr1,3218610,154074261,*,69643,-0.122,YES
chr1,154080441,154081058,*,3,-13.462,YES
chr1,154089408,247813706,*,59364,-0.1003,YES
chr1,61735,2061969,*,379,-0.326,NO
chr1,2069681,2071738,*,2,-25.242,NO
chr1,2074104,3130590,*,333,-0.3957,NO