plot.DRMod fails when dose or resp arguments for fitMod are supplied as expression

Modified fitMod example where dose is supplied as expression (instead of supplying variable) and plotting fails:

data(IBScovars)
anovaMod <- lm(resp~factor(dose)+gender, data=IBScovars)
drFit <- coef(anovaMod)[2:5] # placebo adjusted estimates at doses
vCov <- vcov(anovaMod)[2:5,2:5]
## instead of creating new dose variable directly use expression in fitMod
#dose <- sort(unique(IBScovars$dose))[-1]
gfit2 <- fitMod(sort(unique(IBScovars$dose))[-1], drFit, S=vCov, model = "emax", type = "general",
               placAdj = TRUE, bnds = c(0.01, 2))
plot(gfit2, CI=TRUE, plotData = "meansCI")  ## fails

This is likely an issue with plotFunc which tries to build a formula based on the original expression which fails (or can have unintended consequences) for expressions such as the one above