Aerochino overflow

When I make a cappuccino I fill my aerochino 3 to the max line and press the button once. After a little while the lid pops off and milk starts overflowing.

I have tested it here with just water. I'm under the max line and it's just about reached the top. If this was milk the foam would be overflowing.

I'm using standard semi-skimmed milk.

What am I doing wrong?