How can I solve this Mockito MissingMethodInvocationException using when()...?

In Eclipse, using junit and Mockito. I'm trying to test that a method returns a List of certain size, but get the following error:

org.mockito.exceptions.misusing.MissingMethodInvocationException: when() requires an argument which has to be 'a method call on a mock'. For example: when(mock.getArticles()).thenReturn(articles);

Also, this error might show up because: 1. you stub either of: final/private/equals()/hashCode() methods. Those methods cannot be stubbed/verified. 2. inside when() you don't call method on mock but on some other object.

My test:

class ValidateBookResultsTests {

public static ArrayList<Book> searchResults = new ArrayList<Book>();
public static List<Book> topFive;
public static ArrayList<String> authors = new ArrayList<String>();

public static Book book1;
public static Book book2;
public static Book book3;
public static Book book4;
public static Book book5;
public static Book book6;

@BeforeEach
public void setUp() {
    book1 = new Book("title1", authors, "publisher1");
    book2 = new Book("title2", authors, "publisher2");
    book3 = new Book("title3", authors, "publisher3");
    book4 = new Book("title4", authors, "publisher4");
    book5 = new Book("title5", authors, "publisher5");
    book6 = new Book("title6", authors, "publisher6");

    searchResults.add(book1);
    searchResults.add(book2);
    searchResults.add(book3);
    searchResults.add(book4);
    searchResults.add(book5);
    searchResults.add(book6);
}

@Test
public void returnFiveBooksFromSearchResults() {
    authors.add("John Doe");
    authors.add("Bill Gates");

    BookSearch mockBookSearch = Mockito.mock(BookSearch.class);
    Mockito.when(mockBookSearch.returnFiveBooks(searchResults)).thenReturn(topFive);

    System.out.println("return books: " + mockBookSearch.returnFiveBooks(searchResults));
    System.out.println("top: "+ topFive);

    assertEquals(topFive.size(), 5);
 }
}

My relevant code:

public static List<Book> returnFiveBooks(ArrayList<Book> searchResults) {
  Collections.shuffle(searchResults);
  topFive = searchResults.subList(0, 5);

  printFiveResults();

  return topFive; }

I've read other solutions that say to create a mock class/object, which I believe I've done with "BookSearch mockBookSearch = Mockito.mock(BookSearch.class);"

What am I missing?