It looks like Vincent was correct that `datagrid()` doesn't work for imputed objects.

I think you have to do this instead:

data_age50 = complete(data_imputed, 'long', include = TRUE) %>% mutate(age=50) %>% as.mids()
with(data_age50, avg_comparisons(my_model, variables = list(Allocation= c("Control", "Intervention")), type = "link"))