Zero could be a perfectly "valid" value and still break your application if changing the function to always return zero breaks an implicit contract. Suppose the

Zero could be a perfectly "valid" value and still break your application if changing the function to always return zero breaks an implicit contract. Suppose the previous behavior of the function returned something you could use as an ID for some internal representation in a database that you could look up and write to. Before, the ID you got back depended on the input, but now it doesn't. And you can't tell you're not getting real IDs back anymore, so you just happily write to the database. Now you're corrupting your database because the implicit behavioral contract you were relying on was broken - I'd MUCH rather the program just crash than for it to continue "working" in a way that irrecoverably corrupts data.