Access 2007 Query

Ive Made a query and for one of the columbs i want to check if the data exists ont eh table if so then it will display that data if not then it will calculate the value between two dates, it works except for getting the value of the table data, all the date calulation and the check seems to work its just the second part of the iif that should display the code from the table [Client Data].[Number of Weeks] and botht eh names are correct but it jsut outputs #Error when it should output the value.

Number of Weeks: IIf([Client Data].[Number of Weeks] != "", [Client Data].[Number of Weeks], DateDiff("ww", [Client Data].[start Date], DateAdd("d", 8-Weekday([Client Data].[Expected End Date], 2), [Client Data].[Expected End Date])))

Any help would be appriciated, thanks.