Cannot read property 'top' of null

I'm console.logging the offset() of an element and I get

left: 1236
top: 139
__proto__: Object

But when I log offset().top I get the error

Cannot read property 'top' of null

Here's my code in its entirety:

console.log(('.index-title').开发者_如何学编程eq(n).prev().offset().top);

If n's 0, then you're at the start of the list and .prev() has nowhere to go, returning null instead of what would have been the "previous" object in the list.