nlme fails if there is an implicit conversion to factor

[pbreheny]

As in the following example, from Marcos Fernández Martínez:

library (visreg)
library (nlme)
dat <- data.frame(group=rep(LETTERS[1:4], 5), y=rnorm(20, 100, 5), x=rnorm(20, 30, 1))
mod <- lme(y ~ x, data=dat, random= ~1|group, method="ML")
visreg(mod)