If your code runs on a different machine, it's normal. The null stream ("A Stream with no backing store" from the documentation) is not marshaled differently over the wire. On the client side, it's just a stream which always returns EOF.

If you need to carry some extra information, you need to find another way (other parameters /value information, exceptions), but not use the Stream itself, or use a data convention understood by both parties. For example "if the file is only 1 byte long and contains the character 255, then, it's the 'null stream'".