Zero is not singular. Only one is singular. It's literally part of the word: singular, single, one.
There is one pen. There are two pens. There are zero pens.
It gets a tad confusing when dealing with fractions, though. There are 0.5 pens. There is half a pen. The second is saying "half of one pen", so it's singular, while the first is providing a quantity that is not singular. If you said "there are two thirds of a pen" the pen is singular while the number of thirds is plural.