[R] gam

[I.Szentirmai]

Dear R users,

I'm new to both R and to this list and would like to get 
advice on how to build generalized additive models in R. 
Based on the description of gam, which I found on the R 
website, I specified the following model:
model1<-gam(ST~s(MOWST1),family=binomial,data=strikes.S),
in which ST is my binary response variable and MOWST1 is a 
categorical independent variable.

I get the following error message:
Error in smooth.construct.tp.smooth.spec(object, data, 
knots) :
         NA/NaN/Inf in foreign function call (arg 1)
In addition: Warning messages:
1: argument is not numeric or logical: returning NA in: 
mean.default(xx)
2: - not meaningful for factors in: Ops.factor(xx, 
shift[i])

I would greatly appreciate if someone could tell me what I 
did wrong. Can I use categorical independents in gam at 
all?

Many thanks,
Istvan