UPDATE: Ok, I made this work using a public variable - which I didn't like to do - but I would still like to understand it. I think it's because THIS is the DE, not the report itself. Is there any way to reference properties of the DE in an expression in the report?
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I have this in the INIT of the DE of a report form:

THIS.AddProperty('TotalCodes')
* Run some SQL...
THIS.TotalCodes = _TALLY

Then this in an expression in the report:

THIS.TotalCodes

I'm getting an error when running the report that TotalCodes is not found. What am I doing wrong? I have other properties that work, though none that are expressions. Is it a scoping thing of some sort?