Error when generating marginal effects from brms model of the form y ~ 0 + intercept + x

  • Operating System: Windows 10
  • brms Version: 2.9.0

Hi all,

I am trying to obtain marginal effects using emmeans and tidybayes from a brms model with a prior on the real intercept and thus a model of form y ~ 0 + intercept + x. However, this results in the following error:

Error in vcov(object)[nm, nm, drop = FALSE] : subscript out of bounds
Called from: emm_basis.brmsfit(object, trms, xlev, grid, ...)

Here’s a quick example (with mostly unchanged defaults) using the warpbreaks data set.



# All seems to works here:
Model1 <- brm(breaks ~ tension,
              data = warpbreaks)

Model1 %>%
  emmeans(~ tension) %>%
  gather_emmeans_draws() %>%

# A tibble: 3 x 7
# tension .value .lower .upper .width .point .interval
# <fct>    <dbl>  <dbl>  <dbl>  <dbl> <chr>  <chr>    
# L         36.4   30.9   42.0   0.95 mean   qi       
# M         26.2   20.6   31.7   0.95 mean   qi       
# H         21.6   16.1   27.2   0.95 mean   qi 
# Error occurs here:
Model2 <- brm(breaks ~ 0 + intercept + tension,
              data = warpbreaks)

Model2 %>%
  emmeans(~ tension) %>%
  gather_emmeans_draws() %>%

#Error in vcov(object)[nm, nm, drop = FALSE] : subscript out of bounds
#Called from: emm_basis.brmsfit(object, trms, xlev, grid, ...)

It seems the error has to do with how the internally generated variable intercept is handled or else there might be a misunderstanding on my part wrt how things should work. Your help is very much appreciated.

This looks like a problem in emmeans. At least, the error seems to be happening inside there. you may need to open an issue at to get some feedback on this problem.

Thanks - I’ve just done this here