NoMethodError: undefined method `string' for File

I've done a debug and I found that TMail::Encode.add_lwsp method here
====>>> fold if restsize() <= 0 <<<===

call its fold method which try to use string method for File class here:

   def fold
     # puts '---- fold ----'
====>>>   unless @f.string =~ /^.*?:$/ <<<=====
       @f << @eol
       @lwsp = SPACER
         .
         .
but @f is a File object that has no string method.

What should I do on order to solve this issue?

Thank you

Laszlo Kollar