Why I'm getting this "Range Violation" error?

[rempas via Digitalmars-d-learn]

I have the following code:

```
import core.stdc.stdio;
void print(T)(string prompt, T args...) {
  size_t len = prompt.length;
  size_t i = 0;

  while (prompt[i] != '{' && i < len) {
    printf("%c", prompt[i]);
    i++;
  }
}

void main() {
  print("Hello, world!\n", 10);
}
```

When I execute it, I'm getting a range violation error. If I try 
to set "len" to be the length of the "prompt" minus 1, then it 
will work and it will print the "prompt" until the questionmark. 
So I cannot find where the error is...