[Twisted-Python] why can't a callback be called with a deferred?

Hi All, There's this assert: https://github.com/twisted/twisted/blob/trunk/src/twisted/internet/defer.py#... ...and I'd like to understand why it's there. I have what feels like a legitimate case where that assert trips me up, as I'd like to change this line: https://github.com/cjw296/carly/blob/d39e316aa0f9e05613c263410e7b3ba5bcacba3... ...to just return result.value. However, if the method being hooked returns a deferred, then I trip the assert. Unfortunately, the commit that introduced that assert is just a one liner from Itamar back in 2003 without any context of what problem lead him to introduce the change. Of course, I can work around it, was more curious than anything... Chris