City-state Will Not Die

[Greenguy90]

I am playing as the Aztecs on easy for fun. I have decided to wipe out all city-states along with the major civs but Stockholm (the last remaining CS) will not be destroyed. I have attacked them twice with 2 bombers and a destroyer and they were at low health. I attacked with my helicopter gunship to capture them, but instead it just shows their health bar as empty and they are still alive. Is it impossible to kill all city-states? Does the heli gunship not count as an actual melee unit?

 

[SarahEspresso]

It has to be a ground-based melee unit or a melee ship. Helicopters can't capture cities.