It only relies on unit tests. The people can fail all day long and the unit tests will catch it every single time. Not special unit tests that attempt to seek out such issues, the same unit tests you are writing in languages that have a stricter type system.

If you forget to initialize a field and the tests don't notice, you didn't need the field in the first place, so it won't matter if it is left in an invalid state.

You just don't get the squiggly lines in your text editor. That's the tradeoff.