Correct. I'm pretty sure the other way to reset a field is to use a 'sql' based distribution rule that has null.For example select '' as 'nullified' and distribute into the required field. This should reproduce it.My other suggestion was really aroun...

From memory it is a little difficult to manage nulls.THe way i've done it in the past (and it will depend on the data type of the field).Physicall set, and detect for the 'three hyphens'.i.e., define the null as how it is displayed in the actual fiel...