How to pass multiple parameters in PowerMockito.doReturn

This is my class:

public class Service {

@Autowired 
Class1 class1;

  public String method1 (){
  Class2 class2= class1.anotherMethod(s1, s2)    //Both String. **Error Line**
  //Do something
  return result          //Returns a string based on something 
 }
}

I have mocked it in my TestClass like:

Class2 class2= new Class2();
Class1 class1 = new Class1();
Class1 class1Spy = PowerMockito.spy(class1);
PowerMockito.doReturn(class2).when(class1Spy, "anotherMethod", Mockito.anyString(), Mockito.anyString());

Now when I call method1 it is throwing NullPointerException in the line mentioned as 'Error Line'. Can anyone help me? I guess I am passing the parameters in Powermockito.doReturn in a wrong way.