Blank slots in enchantment table

(Sorry, I've been posting way too much in this channel, but I've been really into working on this datapack lately.)
I recently made it so curses are included in the in_enchanting_table tag (because I think it would be kind of cool if you could accidentally get a curse by enchanting your tools, especially if I add a ton of unique curses) but sometimes, especially when using an enchanting table with very few book shelves around it, there will be a blank slot in the table (as shown in the attatched image) I'm not sure if this has to do with my datapack, but it'd be helpful to know if this is normal (and if there's a fix).