[R] glm error: factor levels

[Birgitle]

Hello R-Users, again me!

I have a data.frame with 291 rows, 82 columns. 
Tha variables in the columns are factors, numerics and ordered factors.

The response variable is a factor with two levels.

I would like to find the best model by trying every possible variable
combination using a logistic regression and comparing the AIC (still have to
find out how to do this fast and easy).
I am aware of the step function but would like to do it also without step.

But unfortunatley I have allready a problem with the first model using all
variables:

   df.glm<-glm(RespVar~.,family="binomial",data=df)

I get back this:

   Fehler in `contrasts<-`(`*tmp*`, value = "contr.treatment") : 
   Kontraste können nur auf Faktoren mit 2 oder mehr Stufen angewendet
werden

   Error in `contrasts<-`(`*tmp*`, value = "contr.treatment") :
   contrasts can only be used for factors with two or more levels

There are no variables with less than two levels in my data.frame but here
are missing values (NA).
I could imagine that some observations are deleted due to missingness and
perhaps after that, some of the variables contain only one level.

I would be happy if somebody could tell me if my speculation could be true
and perhaps also how I could find out which variables I have to delete to
avoid this problem.

Many thanks in advance.

B.


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