Why does ggplotly break when ggtern is loaded?

Question

I was hoping to use plotly::ggplotly() to plot a graph produced with ggtern() with a tooltip feature. But loading the package ggtern caused ggplotly to throw the following error:

Error in inherits(theme[[element]], ggplot_global$element_tree[[element]]$class) : 'what' must be a character vector

What's more, this error appears for all graphs, not just those produced by ggtern.

Here is an example.

library(ggplot2)

# This works
gg.mtcars <- ggplot(mtcars, aes(hp, mpg)) +
  geom_point()
plotly::ggplotly(gg.mtcars)

library(ggtern)

data("Feldspar")

gg.feldspar <- ggtern(data = Feldspar, aes(x = Ab, y = An, z = Or)) +
  geom_point()

# This throws an error
plotly::ggplotly(gg.feldspar)

# And now this throws an error too
plotly::ggplotly(gg.mtcars)