//in the tester
XCTAssertNotNil(testList.head.next);

failed: ((testList.head.next) != nil) failed: throwing "-[__NSCFNumber next]: unrecognized selector sent to instance 0x8d75720"

- (void)add:(NSObject *)node {
    Node *newNode = [[Node alloc] init];
    if (self.head)
        newNode.next = self.head;
    else
        self.head = newNode;
    self.num_nodes++;
}


NList *testList = [[NList alloc] initWithSize:2];

@property (nonatomic,readwrite) NSInteger size;

.....

- (id) initWithSize:(NSInteger *)size {
    self = [super init];
    if (self){
        self.head = nil;
        self.size = *size;
        self.num_nodes = 0;
    }
    return self;
}

- (void)testAdd
{
    NList *testList = [[NList alloc] initWithSize:2];
    NSObject *testNodeOne =  @1;
    [testList add:(testNodeOne)];
    XCTAssertNotNil(testList.head);
    NSObject *testNodeTwo =  @3;

    [testList add:testNodeTwo];
    XCTAssertNotNil(testList.head);
    XCTAssertNotNil(testList.head.next);

}

/LinkedListTest.m: test failure: -[LinkedListTest testAdd] failed: ((testList.head.next) != nil) failed: throwing "-[__NSCFNumber next]: unrecognized selector sent to instance 0x8d75720"

-(id) initWithSize:(NSInteger)size