Thanks for the suggestion, Tommy. Yes, tried that, no difference. I'm puzzled insofar as the example given used a scope variable, and that supposedly works. I tried to compute the variable name outside the main expression like this

and that didn't work either. I've used a bunch of print statements to verify the field name binding is correct, which it is, at least the print outputs the expected field name. This has me stumped - any suggestion is appreciated.