Does ISBN Verifier's X must be the last one of the isbn string?

Exercism
1

fn invalid_isbn_with_invalid_X() { assert!(!is_valid_isbn("3-598-2X507-9")); }

just a title describe, whey X in the middle of ISBn string is not valid?

below is my code passed the exercise

pub fn is_valid_isbn(isbn: &str) -> bool {
    let mut cnt = 10;
    let mut sum = 0;
    for ch in isbn.chars() {
        match ch {
            '0'..='9' => {
                let n = ch.to_digit(10).unwrap_or(0);
                sum += n * cnt;
                if cnt < 1 {
                    return false;
                }
                cnt = cnt - 1;
            }
            'X' => {
                if cnt == 1 {
                    sum += 10 * cnt;
                    if cnt < 1 {
                        return false;
                    }
                    cnt = cnt - 1;
                } else {
                    return false;
                }
            }
            _ => continue,
        }
    }
    sum % 11 == 0 && cnt == 0
}
2

It’s right in the description:

The ISBN-10 format is 9 digits (0 to 9) plus one check character (either a digit or an X only).

3

Thanks, it’s my mistake.