Greta_mcmc_list not working with saveRDS?

I want to save a mcmc() output so I do not need to rerun it next time. However, using saveRDS() (or save()) seems to decouple the greta_mcmc_list object from other greta_arrays.

Example code:

First, run this:

library(greta)

alpha <- normal(0, 1)
beta <- normal(0, 1)
sigma <- lognormal(1, 0.1)
y <- as_data(iris$Petal.Width)
mu <- alpha + iris$Petal.Length * beta
distribution(y) <- normal(mu, sigma)
m <- model(alpha, beta, sigma)
draws <- mcmc(m, n_samples = 500)

# save draws as rds so do not need to rerun mcmc every time
saveRDS(draws, file = "draws.rds")

Then restart the session and load the greta_mcmc_list object. This will fail halfway:

library(greta)

draws <- readRDS(file = "draws.rds")

# the codes below breaks 
petal_length_plot <- seq(min(iris$Petal.Length),
                         max(iris$Petal.Length),
                         length.out = 100)
mu_plot <- alpha + petal_length_plot * beta
mu_plot_draws <- calculate(mu_plot, values = draws)
mu_est <- colMeans(mu_plot_draws[[1]])
plot(mu_est ~ petal_length_plot, type = "n",
     ylim = range(mu_plot_draws[[1]]))
apply(mu_plot_draws[[1]], 1, lines,
      x = petal_length_plot, col = grey(0.8))
lines(mu_est ~ petal_length_plot, lwd = 2)

You should get an error:

Error: the target greta arrays do not appear to be connected to those in the greta_mcmc_list object

Did I do something wrong?