[R] Runtime Error with multinom

[Andreas Wittmann]

Dear R-users,

i try to fit a multinomial model in order to get an imputation for a 
missing value in factor1.

library(nnet)
factor1 <- factor(c("a","b","c","d"))
factor2 <- factor(c("e","f","g","h"))
size <- c(3,8,2,1)

factor1[3] <- NA
Z<-ifelse(is.na(factor1), 0, 1)

assign("data", cbind.data.frame(factor1,factor2,size),pos=1)
multinom(formula(data),data=data[Z,])

when entering the last line i get a runtime error and R crashes down.

my system is windows xp and R 2.9.1, i tried it also with ubuntu 9.04 
and R.2.8.1 but i get a quite similar error.

Many thanks if anyone could tell me what i do wrong and what is the 
problem here.

best regards

Andreas