Negative credit balance????

I was playing a game, about 30 min in, was building up an asteroids defenses when I noticed I had a negative credit balance and my positive income was actually making the amount less negative. It got back to the positive, but it was really strange to see a negative balance. I hadn't lost anything up that point, this was still at the scramble to colonize stage. I didn't know that sins COULD have a negative credit balance. I could still add structures to the build queue, so the game wasn't stopping me from spending, it just went into negative credits. I forgot to save at that point so I don't have proof, unless one of the stats files keep a running log. Did I find a bug or feature?

pek