Warnings BootMer bootstrap glmer lme4

Dear all,

Currently, I am using the BootMer function to bootstrap my final model (fitted with GLMER from the lme4 package) to estimate the confidence intervals and p-values. Since I am alternating my reference categories, I have to bootstrap several models, and now the bootstrap of one model (below) returns with 50 or more warnings. Is there a standard procedure to follow when this happens? Should I trust the bootstrap when warnings pop up?

I am fitting a binomial model with the (centered and treatment coded) predictors c.con.tr (3 levels), c.type.tr (2 levels), and c.diff.tr (2 levels), and the dependent variable ?contrast? (1 = mentioned; 0 = not mentioned).

Details about the model I want to bootstrap, the bootstrap function and the warnings are presented below:

Model:

m0bcc <-glmer (contrast~c.con.tr*c.type.tr*c.diff.tr+(1+c.type.tr+c.diff.tr|id)+(1+c.con.tr+c.type.tr|item.new), data=mydata, family=binomial (link='logit'), control=glmerControl(optimizer="bobyqa"))

Bootstrap function:

FUN <- function(fit) {
  return(fixef(fit))
}

boost <- function(mdl){
  name <- match.call()$mdl
  boosted <- bootMer(mdl, FUN, nsim = 100)
  save(boosted,file=paste(name,"_boosted",sep=""))
}

p_boosted <- function(mdl,boosted){
  for (n in 1:length(fixef(mdl))){
    print(fixef(mdl)[n])
    (bMCI <- boot.ci(boosted, conf = c(0.95, 0.99, 0.999), index=n, type="norm"))
    print(bMCI)
  }
}

Warnings: ( posted the first 10 warnings, in total there were 50 (or more?).

mdl <- m0bcc