Undocumented String.compareTo(null) NPE?

The following little test throws an NPE:

public class Test {

    public static void  main(String[] args) {

        String a = "a";
        String b = null;

        System.out.println(a.compareTo(b));

    }

}

Yet, the Javadoc of compareTo() does not mention that the parameter cannot be null. This is strange, since Javadocs usually mentions when parameters cannot be null.

Is this just a glitch in the documentation or is there a more fundamental reason/twist I am missing?