geom_edge_arc with facet_edges and caps fails mysteriously.

[Ax3man]

Hi Thomas,

While updating phylopath to work with the upcoming releases of ggplot2 and ggraph I've run into a mysterious problem. A simple reproducible example:

gr <- tidygraph::as_tbl_graph(highschool)

ggraph(gr) +
    geom_edge_arc(end_cap = rectangle(), start_cap = rectangle()) +
    facet_edges(~year)

This fails with the error:

Error in grid.Call.graphics(C_lines, x$x, x$y, index, x$arrow) : 
  invalid hex digit in 'color' or 'lty'

This used to work previously.

It seems that with geom_edge_link, no caps, or no facets, it all works. I have no idea where this comes from. Any ideas?