TypeError：undefined在使用PayPal时不是函数

Question

我在使用此处提到的示例示例时遇到以下错误：https://github.com/paypal/PayPal-node-SDK/blob/master/samples/subscription/billing_plans/create.js。

        return callback(null, billingPlan.id);
               ^ TypeError: undefined is not a function
at /home/mukul/scrub/server/payments/paypal/paypal.controller.js:241:17
at IncomingMessage.<anonymous> (/home/mukul/scrub/node_modules/paypal-rest-sdk/lib/client.js:131:13)
at IncomingMessage.emit (events.js:129:20)
at _stream_readable.js:908:16
at process._tickCallback (node.js:355:11)

以下是我的代码：

function createBillingPlan(planDesc, planAmount, planFrequency, planName, planCurrency, callback) { 
   var billingPlanAttributes = {
    "description": planDesc,
    "merchant_preferences": {
        "auto_bill_amount": "yes",
        "cancel_url": CANCEL_URL,
        "initial_fail_amount_action": "continue",
        "max_fail_attempts": "1",
        "return_url": RETURN_URL
    },
    "name": planName,
    "payment_definitions": [
        {
            "amount": {
                "currency": "USD",
                "value": 25
            },
            "charge_models": [
                {
                    "amount": {
                        "currency": "USD",
                        "value": 2
                    },
                    "type": "SHIPPING"
                },
                {
                    "amount": {
                        "currency": "USD",
                        "value":27
                    },
                    "type": "TAX"
                }
            ],
            "cycles": "0",
            "frequency": frequency,
            "frequency_interval": "1",
            "name": planName,
            "type": "REGULAR"
        },
        {
            "amount": {
                "currency": "USD",
                "value": "0"
            },
            "cycles": "1",
            "frequency": "MONTH",
            "frequency_interval": "1",
            "name": "Trial",
            "type": "TRIAL"
        }
    ],
    "type": "INFINITE"
};

paypal.billingPlan.create(billingPlanAttributes, function (error, billingPlan) {
    if (error) {
        console.log(error);
        return callback(error);  // This is causing error
    } else {
        console.log("Create Billing Plan Response");
        console.log(billingPlan);
        return callback(null, billingPlan.id); // this is causing error
    }
});

} 我知道如何解决这个问题？

由于