from the documentation here:

auto_unbox: automatically unbox() all atomic vectors of length 1. 
It is usually safer to avoid this and instead use the unbox() 
function to unbox individual elements. 
An exception is that objects of class AsIs (i.e. wrapped in I()) are not
automatically unboxed. This is a way to mark single values as length-1 arrays.

Have you tried wrapping your field with class AsIs ?