Ergative or Vocative?

Question.  I'm working on some basic grammar in Tech, whatever I can at this
time.  So far I have a few noun cases worked out, and I think I'm going to
decline nouns according to this scheme:

                'water'     'bride'
Nominative      weat        k@ul        [wE:t, k@ulw]
Oblique stem    weta-       k@lu-       [wEta, k@lu]
Accusative      wetaam      k@luam      [wetam, k@luVm]
Genitive        wetaun      k@luun      [wetaumw, k@lu:n]
Gen. of acc.    wetamaun    k@lumaun    [wetamaunw, k@lumaunw]
Vocative?       weta        k@lu

So for the zero-ending, what would be more likely in a natlang -- a vocative
case or an ergative?

Thanks,

Danny

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