Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
An arithmetic step in rush just failed at the very first moment: reading its own input. Here is how that can happen. Before a builtin adds, multiplies or divides, the engine already checks that the operands are numbers, using the builtin's declared parameter types. Then the step reads each operand back from the heap as an exact decimal or a ratio. That second reading should always succeed, because the first check already passed. So if it fails, either the stored text does not read as a number after all, or the reader could not be given the workspace it needs. Either way, the step has nothing to compute with. Rush will not substitute a zero or guess a value, because a made-up number would flow into everything after it. It stops, naming the step, and the second slot of the message names which reader failed, the exact one or the ratio one. For you, this usually means something unusual happened inside the engine rather than a typo in your script. Please keep the message and report it, so the cause can be tracked down.
Text from the Rush engine's error catalogue.