I also practice TDD, but with a different 'T' - Type Driven Design. I find them much easier to reason about with types and safer (you can't compile your code if it doesn't pass the type check). Just model your data as ADT and pattern matching accordingly.

Of course, types alone can't represent every error cases out there (especially the one related to number or string), so I still write Unit Test for those cases. But the number of Unit Tests needed is much lower.