Bug in plot_stress with which=NULL

library(stressaddition)
model <- ecxsys(
    concentration = c(0, 0.0001, 0.001, 0.01, 0.1, 0.316, 1, 3.16),
    survival_tox_observed = c(84.44, 73.33, 66.67, 53.33, 46.67, 20, 0, 0),
    hormesis_concentration = 0.1
)
plot_stress(model, NULL)

Results in:
Error in endsWith(which, "observed") : non-character object(s)

It works in plot_survival() ad the documentation states "An empty vector or NULL creates just the axes.".