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You receive an error saying a field should not be null when running Duplicate Remover. This is typically caused by a true/false field with nulls. We see this fairly often, particularly on ACT! data bases that started back around ACT! 2006 or 2007 and have since been upgraded to newer versions. Several records have NULL in a YesNo field. You can prove this on your data base by doing a Lookup/All Contacts to get a total record count. Then Lookup/Field/Equals/True and note the count and then Lookup