test.js

///// ERRORS
var stripe = require("stripe")("sk_test_foo")
stripe.customers.cancelSubscription(
    "cus_7kao3kVTFYB3ht",
    "sub_7kaoBQVhkpGlsG",{
        at_period_end_test:true
    },
    function(err, subscription) {
        console.log(err,subscription)
    }
);

///// Works
var stripe = require("stripe")("sk_test_foo")
stripe.customers.cancelSubscription(
    "cus_7kao3kVTFYB3ht",
    "sub_7kaoBQVhkpGlsG",{
        at_period_end:true
    },
    function(err, subscription) {
        console.log(err,subscription)
    }
);