"Ignoring unknown aesthetics: z and zend" but freaks out without z and zend

When using the ggtern library to plot data on the simplex, R keeps giving me the warning

In ggplot2::geom_segment(data = lines, ggplot2::aes(x = x1, y = x2,  :
  Ignoring unknown aesthetics: z and zend

If I remove z and zend, it will give an error

Error in FUN(X[[i]], ...) : object 'V3' not found.

Here's the code for reproducibility

my_matrix <- matrix(c(0.000000, 0.000000, 1.000000,
                      0.666666, 0.000001, 0.333333,
                      1.000000, 0.000000, 0.000000,
                      0.000001, 0.333333, 0.666667,
                      0.333334, 0.333335, 0.333331,
                      0.333332, 0.666668, 0.000000,
                      0.000000, 0.666666, 0.333334,
                      0.666667, 0.333333, 0.000000,
                      0.000000, 1.000000, 0.000000,
                      0.333333, 0.000001, 0.666665),
                    nrow = 10, ncol = 3, byrow = TRUE)

design <- as.data.frame(my_matrix)
lines <- data.frame(x1 = c(0.5, 0, 0.5),
                    x2 = c(0.5, 0.5, 0),
                    x3 = c(0, 0.5, 0.5),
                    xend = c(1, 1, 1)/3,
                    yend = c(1, 1, 1)/3,
                    zend = c(1, 1, 1)/3)
ggtern::ggtern(design, ggplot2::aes(x = V1, y = V2, z = V3)) +
  ggplot2::geom_point(size = 3, color = "blue") +
  ggplot2::geom_segment(data = lines, ggplot2::aes(x = x1, y = x2,
                                                   xend = xend, yend = yend),
                        color = "grey", size = 0.2) +
  ggplot2::theme_bw() + ggtern::theme_nomask() + ggtern::theme_clockwise()

I know the package::function() syntax is super annoying, I'm writing this for a plotting function in a toy R package, hence why there's no library(ggplot2) and library(ggtern).

So, any tips to get this warning to go away? What am I doing wrong? Does this have to do with ggtern being outdated?

I'm using ggplot2 version 3.4.1 and ggtern 3.4.1.

It looks like ggtern overrides geoms' normal mapping and requires each layer to be expressed in x/y/z coordinates where they'd normally just need x/y. So geom_segment needs a z and a zend. Your global aes() in the ggtern line says that z will be found in the V3 column, but lines doesn't have that column, leading to the error. We need to specify where the z and zend will be found in the lines data.

ggtern::ggtern(design, ggplot2::aes(x = V1, y = V2, z = V3)) +
  ggplot2::geom_point(size = 3, color = "blue") +
  ggplot2::geom_segment(data = lines, ggplot2::aes(x = x1, y = x2, z = x3, 
                        xend = xend, yend = yend, zend = zend), 
                        color = "grey", size = 0.2) +
  ggplot2::theme_bw() + ggtern::theme_nomask() + ggtern::theme_clockwise()