The problem with null
The problem with null is that it pretends to be an object of a given type, without actually having that type. For example (C# code - ignore the uselessness of the GetName method): string GetName(Customer customer) { // I got a customer object, I can access the Name property return customer.Name; } var x = func(null); // <-- not a real Customer object so we have a run-time error The proper way to solve this is by using the Option (aka Maybe) monad; for an example using the Functional.Maybe NuGet package: string GetName(Maybe<Customer> customer) { // I don't actually have a Customer object, I have a Maybe<Customer> // I need to treat it carefully return customer.Select(it => it.Name).OrElse("Missing"); } var x = func(null); // compiler error, because Maybe<> doesn't allow null as a value var x = func(Maybe<Customer>.Nothing); // the customer is missing but the call will not crash Using the Opt