quantregForest x不包含数据

运行此代码:

library(datasets)
library(quantregForest)

Xtrain <- iris$Sepal.Length
Ytrain <- iris$Petal.Width

model <- quantregForest(
    x = Xtrain
    , y = Ytrain
)

但收到以下错误消息:

Error in quantregForest(x = Xtrain, y = Ytrain) :  x contains no data

有人能看看出什么事了吗?

附言:

不过,这是可行的:

library(datasets)
library(quantregForest)
library(dplyr)

Xtrain <- iris %>%
    select(
        Sepal.Length
        , Petal.Length
    )

Ytrain <- iris$Petal.Width

model <- quantregForest(
    x = Xtrain
    , y = Ytrain
)