itunes won't let me edit one particular song's track number (the # is in the 100s)

[sun surfer]

So, I can edit track number info for all other songs. In this particular song, I can edit the first field of track number the " # of" part. That's fine. But the "of #" is uneditable. It's already in the hundreds (the number already there is over a hundred so uses all three empty spaces showing in the edit screen, it may even be over a thousand, i don't know because it only shows three spaces). When I click on that area, it will put the blinking cursor there like I should be able to edit it. But it won't let me delete the number already there or put a new one in. What's the problem?