stepAIC handling of multinom models

Question

I am seeing some weird behavior with the stepAIC function in the MASS package when dealing with multinomial logistic models. Here is some sample code:

library(nnet)
library(MASS)
example("birthwt")
race.model <- multinom(race ~ smoke, bwt)
race.model2 <- stepAIC(race.model, k = 2)

In this case race.model and race.model2 have identical terms; stepAIC did not prune anything. However, I need to query certain attributes of the models, and I get an error with race.model2:

formula(race.model)[2]

returns race() but

formula(race.model2)[2]

gives the error:

    Error in terms.formula(newformula, specials = names(attr(termobj, "specials"))) : 
invalid model formula in ExtractVars

This behavior only seems to occur when stepAIC does not remove terms from the model. In the following code, terms are removed by stepAIC, and both models can be properly queried:

race.big <- multinom(race ~ ., bwt)
race.big2 <- stepAIC(race.big, k = 2)
formula(race.big)[2]
formula(race.big2)[2]

Any ideas about what is going wrong here?