I'm trying to use sync function like that:

```
$round->competitors()->sync($fighters);

```

$fighter is : 

```
Collection {#339 ▼
  #items: array:3 [▼
    0 => 5
    1 => null
    2 => 6
  ]
}

```

When it get to the null element, I get :

```
QueryException in Connection.php line 647:
SQLSTATE[22007]: Invalid datetime format: 1366 Incorrect integer value: '' for column 'competitor_id' at row 1 (SQL: insert into `round_competitor` (`competitor_id`, `round_id`) values (, 29))

```

I get the same result if I try:

```
$round->competitors()->sync([null, null]);

```

But if I try:

```
$round->competitors()->attach(null);

```

it works without any problem???

Why is that happening???