How to append a char to a std::string?

The following fails with the error prog.cpp:5:13: error: invalid conversion from ‘char’ to ‘const char*’

int main()
{
  char d = 'd';
  std::string y("Hello worl");
  y.append(d); // Line 5 - this fails
  std::cout << y;
  return 0;
}

I also tried, the following, which compiles but behaves randomly at runtime:

int main()
{
  char d[1] = { 'd' };
  std::string y("Hello worl");
  y.append(d);
  std::cout << y;
  return 0;
}

Sorry for this dumb question, but I’ve searched around google, what I could see are just “char array to char ptr”, “char ptr to char array”, etc.