Code terminated with libc++abi.dylib: terminating with uncaught exception

I am making program that reverses a string and it has to use recursion.

#include <iostream>
#include <string>

void flipper(std::string str) {
    int i;
    if (i == (str.size() - 1))
        std::cout << str;
    else {
        str += str[0];
        str.erase(str[0]);
        i++;
        std::cout << str;
        flipper(str);
    }
}

int main() {
    std::string str;

    std::cout << "What is your string?\n";
    std::cin >> str;

    flipper(str);
}

Whenever I run the code, even if I take out the recursion part and just run

    str += str[0];
    str.erase(str[0]);
    std::cout << str;

It still returns:

libc++abi.dylib: terminating with uncaught exception of type std::out_of_range: basic_string.