Why does my RLE code show std out of range for c++?

Whenever I'm trying to run this program, it always shows me the error message terminate called after throwing an instance of 'std::out_of_range'.

What I figured out, when I try to take input as string then this problem happens. Hence, my loops do not get executed properly.

I really appreciate if anyone can explain me what's wrong with my code!

#include<iostream>
#include<vector>
#include<stdexcept>
#include<string>
using namespace std;

int main()
{
   vector<string> compressed_run_lengths_data;
   vector<char> compressed_characters_data;
   int i;
   int count = 1;
   bool can_be_compressed = false;
   string data;

   try
   {
       cout << "Enter the data to be compressed: ";
       getline(cin, data);



       for (i = 0; i < data.size(); ++i)
       {
            if (!isalpha(data.at(i)))
            {
                throw runtime_error("error: invalid input");
            }
        }

        if (!data.empty())
        {
             i = 1;

              while (i <= data.size())
              {
                   if (data.at(i - 1) == data.at(i))
                   {
                       count++;

                       if (count > 1)
                       {
                          can_be_compressed = true;
                       }
                    }

                   else
                   {
                        compressed_characters_data.push_back(data.at(i - 1));
                        compressed_run_lengths_data.push_back(to_string(count));
                        count = 1;
                   }

                   ++i;
            }

        if (can_be_compressed)
        {
            for (i = 0; i < compressed_run_lengths_data.size(); ++i)
            {
               cout << compressed_run_lengths_data.at(i) << compressed_characters_data.at(i);
            }
        }

        else
        {
           data;
        }

      }

   }

     catch (runtime_error &e)
     {
        cout << e.what();
        return 1;
     }

       return 0;

   }