Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
You computed a key for a map, and the result was not text. Computed keys are allowed, and useful. You can build a key from a value, written in double brackets. But a key is text by definition. So whatever you compute has to be text at the end. Say you have the number seven. You cannot use it directly as a key. Convert it first: to string of seven gives the text seven, and then the key works. You can read it back using m dot, brackets, and the quoted seven. The engine refuses numbers, truth values, null, paths and collections. Notice how the message is written. It tells you the kind, and then the value: not text, number seven. That detail matters, because the quoted seven and the number seven look the same when printed, and the old message could not tell them apart. Now it can. The fix: wrap your computed key in a conversion to text. Text in, key out, and the map takes it without complaint.
Text from the Rush engine's error catalogue.