Minus Gremlins ?

I noticed it yesterday, when I accidentially pressed the button too often, it went to -1... just tried it again, I now have -2
now I'm returning to my hometown... and it's reset to 0
trying to drop some in neighbour cities again... no... doesn't work... no I'm tapping some neighbour's ghosts... got one gremlin... but can only drop this one...
maybe i have to drop the gremblin in another city...