Please also provide the following information in addition to your question:

- Operating System: Ubuntu 18.04.1
- brms Version: 2.5.0

Hello!

I fitted a serie of models using the following reparametrization of gamma distribution :

```
## Define the new family of distribution
gamma2 <- custom_family(
"gamma2", dpars = c("mu", "phi"),
links = c("log", "log"), lb = c(0, 0),
type = "real"
)
## Reparametrize the gamma
stan_funs <- "
real gamma2_lpdf(real y, real mu, real phi) {
return gamma_lpdf(y | mu * mu / phi, mu / phi);
}
real gamma2_rng(real mu, real phi) {
return gamma_rng(mu * mu / phi, mu / phi);
}
"
```

Because I used this formulation to fit both simple and distributional models, I twicked the predict function like this :

```
predict_gamma2 <- function(i, draws, ...) {
if(is.vector(draws$dpars$phi)){
mu <- draws$dpars$mu[, i]
phi <- draws$dpars$phi
y <- draws$data$Y[i]
gamma2_rng(mu, phi)
}
else{
mu <- draws$dpars$mu[, i]
phi <- draws$dpars$phi[, i]
y <- draws$data$Y[i]
gamma2_rng(mu, phi)
}
}
```

Predict function worked perfectly this spring, but returns now the following error message :

```
pred <- posterior_predict(models[[trait_prep[1]]], newdata = NewD, robust = T, allow_new_levels = T)
Error in gamma2_rng(mu, phi) : Expecting a single value: [extent=4000].
```

Do you have any idea what happen??

Thank you!

Lucas