brms version : 2.7.0, installed from github

Hello!

I have trouble reproducing a meta-analysis exemple with brms! Here is a reproducible exemple

```
# Import libraries
library(metafor)
library(brms)
library(tidyverse)
# Formate data
dat <- escalc(measure="ZCOR", ri=ri, ni=ni, data=dat.molloy2014,
slab=paste(authors, year, sep="_"))
dat$sei <- sqrt(dat$vi)/sqrt(dat$ni)
dat$study <- paste(paste(dat$authors, dat$year, sep="_"))
## Formula
brm_form <- brmsformula(yi | se(sei) ~ 1 + (1|study))
## Prior
brm_prior <- prior(normal(0,1), class = "sd")
## Sample from the posterior
brm_out <- brm(formula = brm_form, prior = brm_prior, data = dat,
iter = 500, warmup = 500, cores = 4)
```

The result I get is

```
Erreur : is.atomic(x) is not TRUE
13.stop(msg, call. = FALSE, domain = NA)
12.stopifnot(is.atomic(x))
11.FUN(newX[, i], ...)
10.apply(E, 2, var)
9.throw_sampler_warnings(nfits)
8..local(object, ...)
7..fun(object = .x1, data = .x2, pars = .x3, include = .x4, iter = .x5,
seed = .x6, init = .x7, warmup = .x8, thin = .x9, control = .x10,
show_messages = .x11, chains = .x12, cores = .x13)
6..fun(object = .x1, data = .x2, pars = .x3, include = .x4, iter = .x5,
seed = .x6, init = .x7, warmup = .x8, thin = .x9, control = .x10,
show_messages = .x11, chains = .x12, cores = .x13) at <text>#1
5.eval(expr, envir, ...)
4.eval(expr, envir, ...)
3.eval2(call, envir = args, enclos = parent.frame())
2.run(rstan::sampling, args)
1.brm(formula = brm_form, prior = brm_prior, data = dat, iter = 500,
warmup = 500, cores = 4)
```

Thank you for your help!

Lucas