[R] Strange error message, many thanks

[varin sacha]

Dear R-experts,

Here below my R code giving an error message that I don't understand. If somebody can help me to fix it, it would be highly appreciated.

# # # # # # # # # # # # # # # # # # # # # # # #
install.packages( "robustbase",dependencies=TRUE )
install.packages( "boot",dependencies=TRUE )
library(boot)
library(robustbase)

n<-500
b<-runif(n, 0, 5)
z <- rnorm(n, 2, 3)
a <- runif(n, 0, 5)
df<-data.frame(b,z,a)

y_model<- 0.1*b - 0.5 * z - a + 10
y_obs <- y_model +c( rnorm(n*0.9, 0, 0.1), rnorm(n*0.1, 0, 0.5) )

 # function to obtain MSE
 MSE <- function(data, indices, formula) {
    d <- data[indices, ] # allows boot to select sample
    fit <- lmrob(formula, data = d)
    ypred <- predict(fit)
    mean((d[["y_obs "]] - ypred)^2)
 }

 # Make the results reproducible
 set.seed(1234)
 
 # bootstrapping with 500 replications
 results <- boot(data = df, statistic = MSE,
                  R = 500, formula = y_obs ~ b+z+a)

boot.ci(results, type="bca")
# # # # # # # # # # # # # # # # # # # # # # # # #