Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
A write just bounced, and the reason is that it was empty in a way that matters. Every version in the accretion store has three parts: a label, a being, and a timestamp. The label is the name whose history this version joins. The payload is the actual thing being added. This write was missing one of them, either no label or no payload. With no label, the version has no history to belong to. With no payload, it has nothing to say. So there is nothing to add. Now, why does the engine refuse instead of storing a blank? Because in this store, nothing ever leaves a history. If an empty version were accepted, it would sit there forever, a row no read could ever make sense of. Think of a diary that can never be erased: you do not want to write a blank line in ink. The message gives some context about the write. The fix is on the caller side: make sure every write names its label and carries a real value. Give it both, and the version joins the history cleanly.
Text from the Rush engine's error catalogue.