Try using argument matchers instead:
import static org.mockito.Matchers.*;
//...
boolean result = Whitebox.invokeMethod(testsubject,
"myprivatemethod", isNull(ClassB.class), eq(true));
Once you use Matchers for any of the arguments, you have to use them for all arguments. This is noted in the Mockito documentation here (see subsection Warning on argument matchers).