Sending Laravel Mailable without subject

I'm having an odd issue where it's absolutely required I send a mailable without a subject from my Laravel application.

At the moment, this is the build() in my mailable:

public function build()
{
    return $this->from('example@example.com')
    ->view('emails.shipments.remitShipmentFileImages')
    ->subject(null)
    ->attach(storage_path($this->file));
}

But I've also tried:

public function build()
{
    return $this->from('example@example.com')
    ->view('emails.shipments.remitShipmentFileImages')
    ->subject('')
    ->attach(storage_path($this->file));
}

But when I get the email tests I get 'Process Remit Files' in my subject line. What am I doing wrong?