Text.Story.SetOpenTypeFeature "tnum", 1

When I step through my code and check the currently selected text in a watch with:
CSline(LP).Text.Story.GetOpenTypeFeature("tnum"), the result is 0.
At that point I use:
CSline(LP).Text.Story.SetOpenTypeFeature "tnum", 1
so that (to my way of thinking) should apply the Tabular Lining open type feature.
It does return '1' when checking the getopentypefeature method, however the selected text doesn't change.
Any idea(s) as to what I might be doing wrong?
Thanks in advance for any information you might be able to provide.