Why does Dissonance not work with Manaless Champions?

For some reason, Athene's Unholy Grail's passive "Dissonance" does not give Ability power for % Base Mana Regen on champions without Mana. (Mordekaiser, Shen, Katarina, Etc.) I'm not sure if this is intended or not, would be great to hear back from a developer about this.
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