Format as a number plugin doesn't show value when you don't use any thousands separator

I'm pulling a number (year e.g: 2010) from a remote url.

I'm using the following xpath:

//div[@class="postMeta"]/span/a[@rel="tag"]

It works fine as long as I tamper it to format as a number, but when I remove the option to use a character (let's say ",") as a thousands separator it doesn't show anything.

How come?