So, I went to trade with a recently subjugated enemy, and noticed that they had the option available to request them to declare war on an ally of theirs. I think it was a nation they had a mutual defense pact with.
The strange thing is, the value showed up as negative. I don't recall what the exact value was, but it looked to me to be: -2,147,483,648. I assume this is the number it was, since I think the problem is that the value reached the maximum amount. In this case I asked them to declare war on their friends AND asked for all their money (as luck would have it, they only had about 450k to give me, and nothing else :( ).
So yeah, looks like this is a bug!