Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
A heap door was asked for a member that is not there. Let's see how members are found. Inside the engine, a collection is a row of members, and a door reaches one by its place: the first, the second, or a path of places leading into nested collections. For that to work, three things must be true. The slot must really hold a collection. The place must be zero or more, and not past the last member. And a path must lead through collections the whole way. If any of these fail, there is no member to read or to pour into. Rush could answer with an empty value, or with the nearest member, but both would be made up. So it refuses, and the second slot says which of the three failed. Picture a list of three items, and a request for item number seven. The door says no, plainly. If you hit this in your own script, check the index and the length. If you did not, the defect is in the step that called the door, and it should be reported with the message.
Text from the Rush engine's error catalogue.