Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
This message is about the engine's own bookkeeping, and here is what it means. Some builtins work in two steps. In the first step, the builtin opens its accumulator, a small workspace, and stores the operand on the heap. In the second step, it reads that operand back from the heap and computes with it. That way, what the builtin works on is always what the heap actually holds, and not some hidden private copy. This error says that at the second step, the accumulator held no text. The version the first step should have left behind was not there to read. Rush could compute on nothing and return zero, but that would be an invented answer. Instead it stops, naming the builtin, and the second slot shows the detail. It is like leaving a note for yourself, and finding the note gone when you come back. For you as a script author, this almost always points at an engine defect, not a mistake in your script. Please keep the full message and report it, so the missing step can be traced.
Text from the Rush engine's error catalogue.