Definitely a bug. Good catch. But luckily it is unlikely to affect many people. I'll issue a patch to fix it soon, hopefully this weekend.
The problem is, I believe (I haven't yet had a chance to look closely at the code), each prize has an internal "computed amount" that is the ultimate prize amount. When the prize is set to non-monetary, it has no monetary value, so the "computed amount" is not computed. Unfortunately it appears that when set to non-monetary this amount is also not set to zero as it should be. Thus, if you set a prize to a Fixed amount (say, 100) and then change it to a Non-monetary prize, its internal "computed amount" is still set to 100 and not reset to 0.
To fix this when you see this issue, edit the prize, set its Amount type to "Fixed" and set the amount to 0 (press OK to close the Prize dialog). Then edit the prize again and set it to "Non-monetary".
Again, good catch.