String palindrom giving wrong answer on string.size() but right answer on string.size()-1

This is a program which finds the reverse of a string entered by a user and tell whether its a palindrome or not.... now the issue is this that it is correctly reversing the string when in for loop i initilize with size()-1... but it misses one character when i initialize with size(). why is this so?

  //palindromee
  #include <iostream>
  using namespace std;
  int main()
  {
   string s;
   string s1;

   cout<<"Enter something:  ";
   cin>>s;
   s1.assign(s);
   int k=0;
   for(int i = s.size()-1  ; i>=0 ; i--)
           //why correct ans reversing on size()-1?
   {       // and wrong ans on size()
        s1[k]=s[i];
        k++;
   }
    cout<<"string s= "<<s;
    cout<<"\nstring s1= "<<s1<<'\n';
    int checker=s.compare(s1);
    if(checker == 0)
    {
        cout<<"\n\npalindrome";
    }
    else cout<<"\n NOT A PALINDROME";
   return 0;

}