Help with string subscript out of range error?

I'm very new to c++ and i'm struggling with fixing this string subscript out of range error. It complies but i cannot run it of course.

#include <iostream>
#include <fstream>
#include <string>

using namespace std;

int main() {
    string sentence, word = "";
    int wordCounter = 1;
    ifstream file("secret.txt");
    getline(file, sentence); 
    for (int i = 0; i < sentence.length(); i++) {
        if (sentence[i - 1] == ' ') {
            wordCounter++;
            if ((wordCounter) % 5 == 0) {
                word += sentence[i] - 32;
            }
        }
    }
    cout << word;
    return 0;
}

January is the first month and december is the last. Violet is a purple color as are lilac and plum.