What object to pass to buffer.dissolve_field?

Hi,

I am using ESRI.ArcGIS.AnalysisTools.Buffer to buffer a feature and dissolve on two fields. Does anyone know what type of object I need to pass to the .dissolve_field property? I have tried a list object containing field objects but this throws an error on execution of the buffer.

The buffer works if I disable the .dissolve_option property but I really need to buffer and dissolve fields.

Thanks,
Luke