Question about attribute casting

[blorange2]

In my code I have been doing the following to explicitly set boolean values, the null part is redundant but anyway.

/**
 * Set the attribute or explicity return null.
 *
 * @param [type] $value
 */
public function setSubsidaryRuleCAttribute($value)
{
    $this->attributes['Subsidary_Rule__c'] = boolval($value ?? null);
}

Recently I wondered whether or not because the Lester\EloquentSalesForce\Model extends an abstract model class that itself extends a vanilla Eloquent model, whether you could use the $casts property?

So I tried it:

protected $casts = [
    'Subsidary_Rule__c' => 'boolean',
];

However, when testing I get the following:

Cannot deserialize instance of boolean from VALUE_STRING value 1 or request may be missing a required field at [line:1, column:2]

I'm wondering whether I don't understand $casts or whether this is a bug?

Thanks in advance.