`_fill_with_stored_generator` looks like one of the steps you wrote, likely its a copy & paste leftover. Based on the difference I'd guess it has something to do with the type of element, like if `ele.value` doesn't exist then fall back to assigning to `ele._control.value`

If this is a zope thing as you suggest earlier, then you can try not catching the exception and then re-running using zope (should be available in all salad installs with `--browser zope` or `--browser zope.browser`) and then seeing what exception it throws.