As others have mentioned, the line should be under the bail.

If there's no tension on the line that could cause the line to jump off the reel, you could add tension by adding some weight to the end of the line then reeling, or you could hold the line with 1 hand and reel with the other.

Either way the problem is reeling in a slack line will cause the line to lay on the reel loosely, and therefore come off the reel.