Grouping by variable in geom_link2 throws error

[jlambvo]

With ggplot2::geom_path, setting group = _y_ in the aesthetic mapping connects observations with the same value for y. For example, I am visualizing the paths taken by agents in a 2D environment, where I want to stratify paths by a unique agent ID. However, when I try to to use this argument in geom_link2 I receive the error:

Warning message:
Computation failed in `stat_link2()`.
Caused by error in `data.frame()`:
! arguments imply differing number of rows: 2, 0

A minimal working example using the reference:

lines <- data.frame(
  x = c(5, 12, 15, 9, 6),
  y = c(17, 20, 4, 15, 5),
  xend = c(19, 17, 2, 9, 5),
  yend = c(10, 18, 7, 12, 1),
  width = c(1, 10, 6, 2, 3),
  group = c("a", "a", "b", "b", "c")

 ggplot(lines) +
     geom_link2(aes(x = x, y = y, colour = colour, size = width, group = group),
                lineend = 'round', n = 500)
)

In fact, this works when setting group to any of the columns. It seems to work only if there are at least 2 observation in each group and the same number of observations across groups (2 <= N_i == N_(-i)).

Is this expected, and is there a way to achieve groups as in geom_path?