Groovy will automatically convert integers into objects, as it sees fit. And one such case is when you assign null to an integer.

There's some more languages, which try to treat primitive types like objects, to make them more consistently usable. As I understand, nullability is a big part of the reason why it can't be solved with syntactic sugar, so presumably this would be possible in all those languages.
If I'm not mistaken, Ruby is another one of those languages.

Just cast it. /s

Probably some monad magic idk functional programming.