unable to output string?

[cypher5783]

	void write_on_paper(string words)
	{
		if (InkLevelPercent <=0)
		{
			cout << "Oops! Out of ink!" << endl;
		}
		else
		{
			cout << words << endl;
			InkLevelPercent = InkLevelPercent - words.length();
		}
	}

1>c:\users\noah\documents\visual studio 2008\projects\objsnclasses\objsnclasses\pen.h(40) : error C2679: binary '<<' : no operator found which takes a right-hand operand of type 'std::string' (or there is no acceptable conversion)

The error code is referring to

cout << words << endl;

I'm still learning C++ and this is an example from the book. But it won't compile. can anybody help please? thanks in advance.