Error in if (is.mer(x) && x$stan_function != "stan_gamm4") after updating package version

I would like to raise a question, which is already present but still unsolved here:

After updating rstanarm package to the newest version (2.17.2), I get an error message when trying to load a stanreg object that was fitted with an old version of rstanarm:

Error in if (is.mer(x) && x$stan_function != “stan_gamm4”) return(formula_mer(x, :
missing value where TRUE/FALSE needed

Is there already a workaround?
Thank you in advance!

You have to restimate the model. Or maybe it would work if you first did

m.rstanarm$stan_function <- "stan_glmer" # or whatever you used