And if not, why not?

The following index always fails for me, even though I had thought I could have a sort order with a list property as long as the index didn't sort or match against any other properties.

- kind: Foo
  properties:
  - name: location_geocells
  - name: time
    direction: desc

If such a composite index is allowed, are there any reasons that this might be failing for me? Do the existence of other indices on the same model increase the likelihood of this failure? Does the combination of a sort order with a list property require more than N entries, where N is the number of values in the list property? (If so, how many does it require?)