using Imposition SlugLine

[BetMason]

I need some help I am trying to print at the top of the 1st page of the ouput pdf.

using this rule in on job start
FusionPro.Composition.impositionSheetSlugText = "Store #"+ Field("store #");

I get this error with no information printing at the top of the page

uncaught exception: Error: In Field(), no data source defined or data could not be loaded

I am I missing a line of code?