Whitebox invokeMethod with eq("string")

I have the test

 Document document = spy(new Document());
    Whitebox.setInternalState(documentReceiverInteractor, "document", document);

    String text= "string";

    Whitebox.invokeMethod(documentReceiverInteractor, "saveFields", anyString(), eq(text), anyString(),
            anyString(), anyString(), anyString(), anyString());

    verify(document).setText(text);

after startup, I get this error:

Argument(s) are different! Wanted: document.setText( ); -> at DocumentReceiverInteractorTest.saveFields(DocumentReceiverInteractorTest.java:98) Actual invocation has different arguments: document.setText( null ); What should I do?