OG Professional - Parent property must be set - Data Import Error

I am trying to import gradation data into a project into the group Particle Size Distribution - Data. This group is a child group to Particle Size Distribution - General. When attempting to import the data, I run into the following error: 

I've checked the spelling on all the headers within the csv file to confirm that is not the issue. I have no problems importing other groups that don't have parent/child relationships. Has anyone run into this error before and know how to resolve it? 

Thanks,