Can't create std::vector of std::ofstream objects

I'm trying to create a vector of ofstream but it doesn't work.

std::vector<std::ofstream> files(10);

The program stops working after this line.

auto createFiles(int n)
{
    std::vector<std::ofstream> files;
    for(int i=0; i<n; ++i)
    {
        std::string fileName("Card" + toString(i+1) + ".txt");
        files.emplace_back(fileName);
        if(!files[i]) throw std::runtime_error("Could not open file " + fileName);
    }
    return files;
}

I've tried this way but it doesn't work either. (the program stops working after the first execution of files.emplace_back(fileName);)