C++ - Error - No operator "[]" matches these operands

I'm working on code for a container that stores strings and sorts them in alphabetical order (thought that it'd be a fun idea). I've been attempting to put a "[]" operator and assign it to the private member words so I can access any data or strings inside of said member. However, I've been struggling with this continuous error that I'm having trouble fixing. It says:

No operator "[]" matches these operands. Operand types are std::shared_ptr(std::vector(std::string, std::allocator(std::string)))[size_t] (Pretend the parentheses are angular brackets)

Here's some of the code regarding the error (Error is present at class.cpp):

class.h

#pragma once

#include <memory>
#include <vector>
#include <string>
#include <iostream>


class sort
{
public:

//...

    std::shared_ptr<std::vector<std::string>> & operator [](size_t st);

//...

private:

    std::shared_ptr<std::vector<std::string>> words;
    std::string alpha = "abcdefghijklmnopqrstuvwxyz";

};

class.cpp

#include "sort.h"

#include <memory>
#include <vector>
#include <iostream>

//...

std::shared_ptr<std::vector<std::string>> & sort::operator[](size_t st) 
{

    return words[st]; //Error is defined at the brackets
}

//...

Another thing to note is that if I remove the brackets with st, the error is gone (Obviously not what I'm trying to achieve). Any help or a fix to this code would be greatly appreciated. Thank you.