Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
The alphabet you chose for a random id has a problem. Here is what a good alphabet looks like. Id dot random picks each character from your alphabet, and every character must have exactly the same chance. For that, the alphabet needs between two and two hundred fifty-six characters, and every one of them must be different. Why different? Suppose you write the letter a twice. Then a would be drawn twice as often as the others, and your ids would lean toward a. That bias is a weakness, especially for ids that should be hard to guess. Rush could quietly remove the duplicate, but then it would be changing what you wrote, and you would not know. So it refuses, and shows you the alphabet in the first slot, with the problem in the second. An alphabet with one character gives you no randomness at all. Remove the repeated characters yourself, keep between two and two hundred fifty-six, and the id will be fair.
Text from the Rush engine's error catalogue.