I found I got this kind of error for a calculated field when the field that I was referencing was not on the report.
e.g. I want to concatinate the First Last and Middle name fields so on my report the fields control source looks like this:
=[firstName] & " " & [MiddleName] & " " & [LastName]
Access requires that the [firstName], [MiddleName] and [LastName] fields be apart of the report. Just make them hidden and no one will be the wiser.
Travis L Abrahamson