R: error when doing backward feature selection with rms::fastbw

I want to perform backward feature selection using the function fastbw from the rms package. I use a sample dataset PimaIndiansDiabetes as below:

library(mlbench)
data(PimaIndiansDiabetes)

library(caret)
trControl <- trainControl(method = "repeatedcv",
                          repeats = 3,
                          classProbs = TRUE,
                          number = 10, 
                          savePredictions = TRUE,
                          summaryFunction = twoClassSummary)

caret_model <- train(diabetes~., 
                     data=PimaIndiansDiabetes, 
                     method="glm", 
                     trControl=trControl)

library(rms)
reduced_model <- fastbw(caret_model$finalModel)

This gives me an error:

Error in fastbw(caret_model$finalModel) : fit does not have design information

May I know what this means and how to resolve it?