R: Write arff with missing values

Question

I'm constructing a data frame on R and want to export it to ARFF format. To do so, I am using the write.arff() function from RWeka library. However, when dealing with missing values (?), the output is like:

@attribute atomic_comp_c {'?',3014,3212,3226,3234,3236,3238,3}

Instead of:

@attribute atomic_comp_c numeric

Does anyone know how can I export the data frame to a ARFF format correctly?

Thanks in advance