Laravel 4 - Eloquent不绑定参数

Question

While trying to add a new row to a database table, I'm getting the following error:

SQLSTATE[23000]: Integrity constraint violation: 1452 Cannot add or update a child row: a foreign key constraint fails (testas.translations, CONSTRAINT translations_ibfk_2 FOREIGN KEY (labelId) REFERENCES label (id)) (SQL: update translations set labelId = where id = 103)

As you can see, it doesn't bind a parameter to labelId. What's causing this?

EDIT:

$labelKey = Label::max('key') + 1;
$label = new Label(array('key' => $labelKey));
$label->save(); //the label gets saved in the labels table


$title = new Translation(array('phrase' => $input['title']));

$title->language()->associate($language);
$title->label()->associate($label);

$title->save(); //this part is failing