I am solving a question on leetcode where I have to multiply strings and I got this error which I cant understand

Line 518: Char 69: runtime error: applying non-zero offset 18446744073709551615 to null pointer (basic_string.h)

SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/basic_string.h:527:69

Here is my code

```
string multiply(string s1, string s2) {
vector<int> vec(s1.length()+s2.length(),0);
for(int i=s2.length()-1;i>=0;i--){
for(int j=s1.length()-1;j>=0;j--){
int pro = (s1[j]-'0')*(s2[i]-'0') ;
//borrow sent to next
int sum = vec[i+j+1] + pro;
vec[i+j+1] = (sum%10);
vec[i+j] += (sum/10);
}
}
string ans="";
for(int i=0; i<(s1.length()+s2.length());i++){
if(ans=="" && vec[i]==0){
continue;
}
ans.push_back(vec[i]+'0');
}
if(ans==""){
return 0;
}
return ans;
}
```

LINK : https://leetcode.com/problems/multiply-strings/

Source: Windows Questions C++

## 2 thoughts on - runtime error: applying non-zero offset 18446744073709551615 to null pointer (basic_string.h)

add this on the first line:

if ( s1.front ( ) == 0 || s2.front ( ) == 0 )

return “0”;

sorry, I mean this:

if ( s1.front ( ) == ‘0’ || s2.front ( ) == ‘0’ ) // here is zero

return “0”;