Laravel 4 - A four digit year could not be found Data missing - database

Question

I'm trying to update some data and I'm having the next message:
A four digit year could not be found Data missing
In this form, I have some data that I want to update in my 'Actividad', so I have created an array with every field I need to update.
But, one of the form fields, I don't want to put it in the same array, because I need it to update the data in a pivot table ('actividad_material').
By the moment, I'm not able to update any of them... :(
Here is my code:
public function update($id)
{
$input = array(
'titulo' => Input::get('titulo'),
'actividad' => Input::get('actividad'),
'datos' => Input::get('datos'),
'solucion' => Input::get('solucion'),
'minutos' => Input::get('minutos'),
'tipo_id' => Input::get('tipo_id'),
'asignatura_id' => Input::get('asignatura_id'),
'bloque_id' => Input::get('bloque_id'),
'contenido_id' => Input::get('contenido_id'),
'objetivo_id' => Input::get('objetivo_id'),
'nivel_id' => Input::get('nivel_id'),
'etapa_id' => Input::get('etapa_id'),
'm_desc' => Input::get('m_desc'),
'slug' => Str::slug(Input::get('titulo')),
'user_id' => Sentry::getUser()->id,
'_method'
);
$v = Validator::make($input, Actividad::$rules);
if($v->passes())
{
// Trying to update my pivot table...
$actividad = Actividad::find($id);
$material_id = array(
'material_id' => Input::get('material_id'));
$actividad->materials->sync($material_id);
Actividad::find($id)->update($input);
return Redirect::route('actividads.index');
}
return Redirect::back()->withErrors($v);
}
Any idea why I'm getting this error? Maybe the timestamps?
What am I doing wrong when updating?
Thank you very much in advance!!