Unexpected String behaviour when trying to copy value from char variable

When i try to assign a char value to a specific position in string i get a blank string as a result.

 while(count<size_s&&count<size_sum)
   {
    int s=s1[size_s-1-count]+sum[size_sum-1-count]+carry-96;
    sum[size_s-1-count]=get_char(s);//get_char is  a way to get char value 
    carry=s/10;
    count++;
   }

expected result would be to get the string with the value that get_char fetches but instead i get an empty string