TypeError：使用PayPal时，undefined不是一个函数

我在使用这里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 } }); 

任何想法如何解决这个问题？

谢谢