[R-pkg-devel] Error appearing only with check_win_devel() - could be ggplot2 R version?

I have a plot example that runs fine on Windows, but in the windows check I
get the following message:

Warning: package 'ggplot2' was built under R version 3.7.0

plot_spec_from_hist(photons_cs137_hist[1:16, ], "mR_h", "Example: Cs-137 with scatter")

Error in valid.units(units) : Invalid unit
Calls: <Anonymous> ... unit.c -> unit -> valid.unit -> structure -> valid.units


Could this be only happening because of the version ggplot2 is built
under in this test? I'm running the latest R version - 3.5.3.


Thanks,

Mark