spotfire input handler for table only shows "None"

I registered a trivial data function with one input and one output.

output <- input

If I set input type to "Value" then when I go to run the function and select inputs/ouputs, under "Input Handler" I get the normal options "Value" and "Document Property".

But if I set the input type to "Table" or "Column" then when I try to run it, under "Input Handler" I only get "None". So I can't select which table to pass in as the input!

The output handler works fine -- I can select new table, etc.

Any ideas on what could be the issue? Here's an example of someone doing the same thing and apparently not running into any problems: http://datashoptalk.com/terr-in-spotfire-passing-marked-data-through-to-another-table/

Spotfire 7.6.0

Ok, I figured this out. The only tables in this instance were database tables, and apparently data functions cannot be used with DB data (spotfire doc).

I guess my next question is ... why? But that can be another topic.