I encountered this problem while interacting with the TestThing. When invoking the "void-void" action, the action results in an 505: Internal Server Error with the message No schema defined. This is thrown inside value() as seen below:

As no data schema is needed for an action with no output, calling value() on the interaction output throws an error. However, this behavior seems a little counterintuitive. I would think, intuitively, that an action without an output DataSchema would either provide no InteractionOutput at all or would provide and InteractionOutput with a value() method that resolves with undefined and an arrayBuffer() method that resolves with an empty array (maybe also undefined but it makes more sense you would actually get an array, but it's empty because there is no output).