I am using the [ApexMocks framework][1] and I am having the following error message:

> Class.MockTest.testBehavior: line 13, column 1 11:53:56.3
> (10709808)|FATAL_ERROR|System.NullPointerException: Attempt to
> de-reference a null object

Anyone with experience with the [ApexMocks framework][1] that can tell me why the test is failing? 

**Class to test:**

    public class fflib_MyList implements IList {
        List<String> myList;
    
        public fflib_MyList() {
            this(new List<String>());
        }
    
        private fflib_MyList(List<String> myList) {
            this.myList = myList;
        }
    
        public void add(String value) {
            myList.add(value);
        }
    
        public String get(Integer index) {
            return myList.get(index);
        }
    
        public void clear() {
            myList.clear();
        }
    
        public Boolean isEmpty() {
            return myList.isEmpty();
        }
    
        public interface IList {
            void add(String value);
            String get(Integer index);
            void clear();
            Boolean isEmpty();
        }
    }

ApexMocks test:

    @IsTest
    private class MockTest {
        static testMethod void testBehavior() {
            // Given
            fflib_ApexMocks mocks = new fflib_ApexMocks();
            fflib_MyList.IList mockList = (fflib_MyList.IList)mocks.mock(fflib_MyList.class);
    
            // When
            mockList.add('bob');
    
            // Then
            ((fflib_MyList.IList) mocks.verify(mockList)).add('bob');
            ((fflib_MyList.IList) mocks.verify(mockList, fflib_ApexMocks.NEVER)).clear();
        }
    }


Source : [FinancialForce ApexMocks Framework][1]


  [1]: https://github.com/financialforcedev/fflib-apex-mocks