Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
You asked rush to split a text using an empty separator, and it said no. Let's see why. Split cuts a text at every place where the separator appears. With a comma, the text a comma b comma c becomes three pieces. Now try an empty separator. Where does nothing appear? Between every pair of letters, and nowhere at the ends. So what should the answer be? A list of single characters? The whole text as one piece? Something else? There is no single right answer, and rush will not guess, because a guess looks just like a correct result. So it refuses by name. The fix depends on what you actually wanted. If you meant to split on a real character, such as a space or a comma, pass that as the separator. If you wanted the individual characters, select them directly with the range form, the text, a dot, and the range of positions, one to n. Be explicit about your intent, and rush will deliver exactly that. If the empty separator came from a variable, check why it was empty. That might be the real bug.
Text from the Rush engine's error catalogue.