Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
You tried to pick a member out of something that has no members. In rush, selection is naming. On a list, you name a position, like x dot two. On a map, you name a label, like x dot age. But if x holds a number or a truth value, there is nothing inside to name. That is a wrong kind of thing, so the engine refuses and tells you the holder and its kind. Now, here is an important difference. If x holds null, that means absence, and selecting from absence answers null, no refusal. And if x is a real collection and you ask for a position past the end, the answer is also null. Those are cases of nothing being there. But a number is not a collection at all. The message shows the name and what it holds. Check where the value comes from. Maybe a step earlier gave you a single value when you expected a list. Fix that, and selection will work as expected.
Text from the Rush engine's error catalogue.