Let's say I have a ball filled with water fixed to a massles rod undergoing oscillations

I've been told that the moment of inertia can be taken as md² ,d being the distance from the pivot

I don't understand why it's not 2mr²/5+md²

Let's say, I have a ball filled with water fixed to a massles rod undergoing oscillations

I've been told that the moment of inertia can be taken as $m d^2$, $d$ being the distance from the pivot

I don't understand why it's not: $\frac{2}{5} mr^2 + md^2 $