When I called Braintree::Transaction.sale, got error "CVV must be 4 digits for American Express and 3 digits for other card types"

Question

When I called Braintree::Transaction.sale with the "payment_method_nonce" which created by Drop-in UI, everything is alright. However, after I added some custom fields in the Drop-in UI and call the same API, I got an error "CVV must be 4 digits for American Express and 3 digits for other card types". I'm quite sure the CVV should be correct... Does anyone have idea on this error?

iOS client side implementation:

    internal func tokenizeCreditCard(creditCard: CreditCard, completion: @escaping NerfireAPICompletionBlock) {

    // For client authorization,
    // get your tokenization key from the control panel
    // or fetch a client token

    let braintreeClient = BTAPIClient(authorization: <sandbox_token>)!
    let cardClient = BTCardClient(apiClient: braintreeClient)

    let card = BTCard(number: creditCard.number!, expirationMonth: creditCard.month!, expirationYear: creditCard.year!, cvv: creditCard.verificationValue!)
    cardClient.tokenizeCard(card) { (tokenizedCard, error) in
        if error != nil {
            completion(nil, error as NSError?)
        }else {
            completion(tokenizedCard, nil)
        }
    }
}

Ruby server side:

result = Braintree::Transaction.sale(
    :amount => "15.00",
    :payment_method_nonce => nonce_from_the_client,
    :options => {
        :submit_for_settlement => true
    }
)