Error: [string "?"]:1973: bad argument #1 to 'check' (number expected, got nil)

[just-mika]

Hello
I've been installing custom themes for a while, but suddenly I got this error.

Error: [string "?"]:1973: bad argument #1 to 'check' (number expected, got nil)

I discovered that whenever I remove the themes folder at the 3ds folder (where its supposed to be) it works perfectly fine, but when I put it back at the 3ds folder, the error shows up again.

I tried putting the themes folder at the root of the sd card and use the "scan for themes at sd" thing but it won't work, so how do I fix this?