Evaluation (NSIntegers) inside if-statement Objective-C

I am in doubt, why this work correctly:

NSInteger row = indexPath.row;
NSInteger preloadTrigger = self.nodes.count - 20;
if (row >= preloadTrigger) {
    [self.loader loadNextposts];
}

And this does not (just skips the if-statement):

if (indexPath.row >= self.nodes.count - 20) {
    [self.loader loadNextposts];
}

when the value of self.nodes.count - 20 is negative.

However, when the value of the expression is positive, it works fine always.

A very strange behavior, as I cannot see semantical difference in two expressions.