C++ runtime error: addition of unsigned offset?

I wrote the following to check if text is palindrome, I run it on leetcode and I am getting errors:

class Solution {
public:
    bool isPalindrome(string s) {
        int l=0,r=s.length()-1;
        while(l<r)
        {
            while (!isalpha(s[r]))
            {
                --r;
            }
            while (!isalpha(s[l]))
            {
                ++l;
            }
            if (tolower(s[r])!=tolower(s[l]))
                return false;
            --r;
            ++l;
        }
        return true;
    }
};

Line 1061: Char 9: runtime error: addition of unsigned offset to 0x7ffc7cc10880 overflowed to 0x7ffc7cc1087f (basic_string.h) SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/basic_string.h:1070:9

what's the problem with my code?