Bayesian information criteria in stan - bell regression model

I’m facing a problem getting the Bayesian information criteria (WAIC and LOO) for the bell regression model in Stan.

I present below the functions necessary to carry out the adjustment of the model already described.

Bell Distribution Functions:

functions
{
    real lambertW(real x)
    {
      real y;
      real w;
      y= sqrt( 1 + exp(1) *x);
      w=-1 + 2.036 * log( (1 + 1.14956131 * y)/(1 + 0.45495740*log(1+y)) );
      w = (w/(1+w)) * ( 1 + log(x/w) );
      w = (w/(1+w)) * ( 1 + log(x/w) );
      w = (w/(1+w)) * ( 1 + log(x/w) );
      return(w);
    }

    real bellnumber(int n)
    {
      if(n < 2){ return(1); }
      else
      {
        int k;
        vector[n] B;
        vector[n] Bneu;
        B[1] = 1;
        for (i in 1:(n - 1))
        {
          k = i;
          Bneu[1] = B[i];
          for (j in 2:(i + 1))
          {
            Bneu[j] = B[j - 1] + Bneu[j - 1];
          }
          for(j in 1:n)
          {
            B[j] = Bneu[j];
          }
        }
        return(Bneu[k + 1]);
      }
    }

    real bell_lpmf(int x, real theta)
    {
      real Bx;
      real lprob;
      Bx = bellnumber(x);
      lprob = x*log(theta) - exp(theta) + 1 + log(Bx) - lgamma(x+1);
      return lprob;
    }

    real loglik_bell(int[] x, real[] theta)
    {
      real lprob[num_elements(x)];
      for(i in 1:num_elements(x))
      {
        lprob[i] = x[i]*log(theta[i]) - exp(theta[i]);
      }
      return sum(lprob);
    }
}

Stan program and accompanying data.


data
{
     int<lower=1> n; // observacoes
     int<lower=1> p; // nº de parametros
     int y[n]; // variavel resposta

     matrix[n, p] X; // Matriz do modelo 
}
parameters
{
     vector[p] beta; 
}
transformed parameters
{
     vector[n] eta = X*beta; // preditora linear
     real mu[n];
     real theta[n]; // Media

     for (i in 1:n)
     {
          mu[i] = exp(eta[i]);
          theta[i] = lambertW(mu[i]);
     }
}
model
{
     for(i in 1:p)
     {
         beta[i] ~ normal(0, 10); // priori 
     }

     target += loglik_bell(y, theta); // Loverossimilhanca incompleta
}
generated quantities
{
  vector[n] log_lik;
  for(i in 1:n){
    log_lik[i] = bell_lpmf(y[i] | theta[i]);
  }
}

The data to be reproduced is in the bellreg package, described below:

bellreg::faults
head(faults)

I present below the codes in R for the adjustment of the model and the error in which I have been facing.

library(bellreg); library(rstan); library(loo)

head(faults)

Modbell = stats::model.matrix(~ lroll, data = faults)

standata = list(y = faults$lroll, X = Modbell ,
                n = nrow(Modbell ), p = ncol(Modbell) )

modestbell = stan(file = "Bell.stan", data = standata, chains = 4, 
                iter = 20000, warmup = 10000, thin = 1, verbose = F)

print(modestbell, pars = c("beta"), probs = c(0.025,0.975), 
      digits_summary = 4)

loo(modestbell)
waic(extract_log_lik(modestbell))

> loo(modestbell)
Error in validate_ll(log_ratios) : All input values must be finite.
> waic(extract_log_lik(modestbell))
Error in validate_ll(x) : All input values must be finite.

How could this problem be solved?

I describe here the link of the article where I have been based to implement the functions: