Angles in data file

Hello guys,
I have a wired problem with angles in the lammps data file.
For example, if I have a water molecule, then the angles should be written as
1 1 1 3 2. (HHO,atomID is 1,2,3.)
Then I add an oxygen molecule under the water, the angles should be
1 1 1 2 3
2 2 4 5.(O2,atomID is 4,5.)
It worked well.
But when I have a system with pure oxygen, it always failed with the message invalid atomID in angles section.
I read from manual that the syntax for angles is ID type atom1 atom2 atom3.
So does it mean that when I write angles in data file, there must be three atoms in an angle?
But if it is, why my water and oxygen data worked well surprisingly?
I know an angle for a two-atom molecule is 180 degree. I tried to delete the angles of oxygen in data file and it also didn’t work.
Can anyone give me some advices?