Junit does not get full coverage for test case

This is my JUnit test case

public void emptyTest() {
        Sort test = new SortList();
         Boolean output = test.isEmpty();
         assertNotNull(output);
         assertEquals(true,output);
        
         
    }

Code i am testing

public boolean isEmpty() {
        return (first == null);
    }

I am currently getting 71% coverage for this test case.