Adapting quantile regression to the caret package

Question

I want to try adpat quantile regression to caret package. I wrote below code to adapt but I get the error as below:

library(quantreg)
quantregression <- list(type='Regression', library='quantreg',loop=NULL)
param <- data.frame(parameter = c("tau"), class = c("numeric"), label = c("tau"))
quantregression$parameters <- param
quantGrid  <- function(x, y, len=NULL, search="grid"){
  if(search == "grid"){
    out <- expand.grid(tau=c(.05, .25, .5, .75, .95)) 
  }
  out
}
quantregression$grid <- quantGrid
quantregFit <- function(x, y, wts, param, lev, last, weights, classProbs, ...){
  dat <- if(is.data.frame(x)) x else as.data.frame(x)
  dat$.outcome <- y
  if(!is.null(wts))
  {
    if (param$tau)
      out <- quantreg::rq(.outcome ~ ., data = dat, weights = wts,tau=tau, ...)
    else
      out <- quantreg::rq(.outcome ~0 + ., data = dat, weights = wts,tau=tau, ...)
  } else
  {
    if (param$tau)
      out <- quantreg::rq(.outcome ~ ., data = dat,tau=tau, ...)
    else
      out <- quantreg::rq(.outcome ~0 + ., data = dat,tau=tau, ...)
  }
  out
}       
quantregression$fit <- quantregFit
quantPred <- function(modelFit, newdata, preProc=NULL, submodels=NULL){
  if(!is.data.frame(newdata)) newdata <- as.data.frame(newdata)
  quantreg::predict.rq(modelFit, newdata)
}
quantregression$predict <- quantPred
quantProb <- function(){
  return(NULL)
}
quantregression$prob <- quantProb

I get the error as below:

Something is wrong; all the RMSE metric values are missing: RMSE Rsquared MAE
Min. : NA Min. : NA Min. : NA
1st Qu.: NA 1st Qu.: NA 1st Qu.: NA
Median : NA Median : NA Median : NA
Mean :NaN Mean :NaN Mean :NaN
3rd Qu.: NA 3rd Qu.: NA 3rd Qu.: NA
Max. : NA Max. : NA Max. : NA
NA's :5 NA's :5 NA's :5
Error: Stopping In addition: There were 50 or more warnings (use warnings() to see the first 50)

warnings() Warning messages: 1: model fit failed for Fold01.Rep1: tau=0.05 Error in unique(tau) : object 'tau' not found

I want to adopt the quantile regression using "quantreg" package for caret to obtain cross validation results for quantile regression. I wrote above code but I could not find the error in the code.