vector subscript out of range

[SkarFaceT9]

Vector subscript out of range error.
any HINTS on why I'm getting this error?

updating this post for a new vector subscript error.
Been trying to debug this for an hour,
it seems that the nextLetter.clear(); is causing the error. But why???

std::string Wordpair::makeSentence(std::vector<std::pair<std::string, char>> completePair, int level)
{
	std::srand(std::time(0));
	std::string finishedSentence;
	std::string endOfString;
	std::vector<char> nextLetter;
	do
	{
		nextLetter.clear();
		if (endOfString.size() > level)
		{
			endOfString.erase(0, 1);
		}
		for (int i = 0; i < completePair.size(); i++)
		{
			if (endOfString == completePair[i].first)
			{
				nextLetter.push_back(completePair[i].second);
			}
		}
		std::random_shuffle(nextLetter.begin(), nextLetter.end());
		endOfString += nextLetter[0];
		finishedSentence += nextLetter[0];
	} while (nextLetter[0] != '.');
	return finishedSentence;
}

I believe I figured it out. It wasn't the clear(),
it was trying to shuffle an empty vector.
a few simple if statements and it was solved.

[Thomas1965]

In lines 19-20 it's possible that nextLetter is still empty.