How do I overwrite fields during an import to a null value?

How do I overwrite fields during an import to a null value?

Hello,

I'm trying to use Item Import / Overwrite to clear the Sales Description field.  I would have expected this to be a simple task importing "Item Name" and "Sales Description" as the two fields by simply leaving "Sales Description" empty (with exception of the header). 

This failed and just left the Sales Description field unchanged.

I then tried populating it with a single space.  This too left the field unchanged.

I then tried using "".  This then changed all the Sales Description fields to now show "".

How can overwrite values with a null value to clear out a field?

Thanks,
Bryan