Run-time error only on someone else's computer

Hi there,

I have created a macro that works 100% of the time on my computer. I've sent it to another computer of mine with a more recent version of Excel and it also works fine. I have also sent the file to three other people. Two of them said it works fine, but there is one person who it doesn't work for. He gets the error: "Run-time error '1004': Application-defined or object-defined error."

Since it never breaks on my computer, I can't really debug anything. I'm wondering why in the world this would happen (only break on one person's computer but not on others), and haven't gotten anywhere on my own, so any help would be greatly appreciated!!