Poison + Despair Interaction

Are Poison and Despair not supposed to stack the same way Petrify and Despair do? With over 20 poison stacks on a regenerating opponent (from Voodoo), their regeneration only goes down to -1. This has happened against various opponents, in act and arenas, against salve and champs with built-in regen.
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If you want to reverse healing, you need a Petrify or Spectre debuff.
On the other hand, with 20 stacks of Poison on your opponent, that shouldn't be a major problem!