[R] Problem with stepAIC, "invalid second argument"

[Klaus Thul]

Dear all,

am I doing something wrong, or is this a bug (R 2.0.1 & 1.9.1,
linux&windows):

    > library(MASS)
    > df <- data.frame(x = rnorm(100), y = rnorm(100), z = rnorm(100))
    > f <- lm(x ~ y + z, data = df)
    > stepAIC(f, scope = list(upper = ~y * z, lower = ~1))
    Start:  AIC= -16.56
     x ~ y + z
    Error in eval(expr, envir, enclos) : invalid second argument
    >


I can avoid the problem doing the following, but in my application I do
not want to use attach / detach because of possible name conflicts:

    > attach(df)
    > f <- lm(x ~ y + z)
    > stepAIC(f, scope = list(upper = ~y * z, lower = ~1))
    Start:  AIC= 23.55
     x ~ y + z

    ...

    Call:
    lm(formula = x ~ 1)

    Coefficients:
    (Intercept)
        -0.1972
    > detach(df)


Thanks for any help,
Klaus Thul