[DFDL-WG] Empty element with initiator and terminator

I have an element that can be one of two things, either: < string > Or <> So I modeled this as a choice. For the first choice, I made it a string with < and > being the initiator and terminator. Works great. For the second choice, I also made it a string with < and > being the initiator and terminator, and I set the length kind to explicit and the length to 0. That seems obvious enough. But, alas, that causes errors in parsing. Nothing I do for this seemingly simple construct works. There must be some design pattern here that I am missing. Ideas? TIA