For any type A -> B operation, it's possible to fail - that is the whole point of parsing into type B, to catch when B's invariants would not be satisfied. The bug could be as simple as neglecting to handle that failure scenario.

Some languages make this less likely (Haskell, Rust) but most mainstream languages will happily let you introduce this bug.

I've caused myself a lot of problems by using/creating types that don't make sense. Coding myself into a corner. So I don't think it's fair to not count type declarations as code.