Date diff accessor always returns null

I've created a date diff accessor in my User model to determine the number of days a user has left in their Cashier free trial. The method looks like this:

Model - returns null

public function getRemainingTrialDaysAttribute() {
        return now()->diffInDays($this->trial_ends_at, false);
}

When I try and access this method in my controller it always returns null, however the same code when written directly in the controller will return the correct date.

Controller - returns integer (correct)

$this->trialDaysRemaining = now()->diffInDays($this->user->trial_ends_at, false);

Can anyone explain to me why it returns null when called as an accessor from the User model?