Add row in each group using dplyr and add_row()

If I add a new row to the iris dataset with:

iris <- as_tibble(iris)> iris %>%     add_row(.before=0)# A tibble: 151 × 5    Sepal.Length Sepal.Width Petal.Length Petal.Width Species<dbl>       <dbl>        <dbl>       <dbl>   <chr>1            NA          NA           NA          NA    <NA> <--- Good!2           5.1         3.5          1.4         0.2  setosa3           4.9         3.0          1.4         0.2  setosa

It works. So, why can't I add a new row on top of each "subset" with:

iris %>%  group_by(Species) %>%  add_row(.before=0)Error: is.data.frame(df) is not TRUE