What is the defined behavior of std::string::erase() with a pos of string::npos?

What does the C++11 standard specify for the behavior of the string& erase (size_t pos = 0, size_t len = npos); member function when the pos argument is passed as string::npos? I would think it should erase nothing, but perhaps it throws an out_of_range exception instead? What is the defined behavior for the standard?