Saxon

The API documentation isn't explicit about what happens when you supply null as the prefix, but it doesn't define any meaning for null, so I think it's a reasonable expectation that null is not an allowed value.

Ideally we should probably be catching Java exceptions and rethrowing them as C# exceptions, but in the general case that would lose stack trace information, so I don't think it would be a particularly beneficial change.

What are you actually trying to achieve? If you are trying to set the default namespace, the prefix should be a zero-length string, not null, as stated in the API documentation.