The minimum amount you can offer for a bounty on a question you have answered is 100. This was added as a small reputation hack was discovered.
However, because the answer you posted has been deleted (and because waffles commented and said "I think I exclude deleted stuff"), I imagine this is a bug.
However (however), in the event of this bug being fixed (allowing you to add a bounty to questions which has a deleted answer of yours in it), the loop hole would be renabled (in the form of deleting answer, adding bounty, undeleting answer). But it's also not fair that you can't add a bounty to your question in this instance.
Indeed, I fail to see what the difference is between adding a bounty to a question which you've got a zero-voted answer on (which you can't do), to adding a bounty to a question and then adding an answer to it (which you can)...