Syntax Error - Cannot Produce Expression as Part of Aggregate Function

[Stevestan1]

I am getting the "cannot produce expression ..... as part of aggregate function" error everytime I try to run a query with this expression

Pricing: IIf([New Run]![Key Account] Is Not Null And [New Run]![Date] <= [Centralization Occurance Dates]![Gl Date],"Pricing","None")

New Run and Centralization Occurance Dates are the tables in the database and date, gl date, and key account are the columns in the tables. Anyone know why this expression isn't passing muster?

[Delerna]

Post the whole query code.
I would say you are using unaggregated fields in your expression