The problem is described here in comments under the answer: https://stackoverflow.com/questions/38338906/nsubstitute-mock-throwing-an-exception-in-method-returning-task

The problem with this is that the exception is thrown at the wrong time.
var t = AsyncMethod(); (actual behavior: exception will be raised here)
await t; (expected behavior: exception will be raised here)

With an async method the exception should be thrown when awaiting the task, Current approach throws the exception immediately.

Current behavior little bit unexpected in some cases. It would be great to correct it. Or may there is more elegant way to solve the problem.