Deleting customer in Sandbox throws TypeError: Cannot read property 'creditCards' of undefined

[BarthesSimpson]

I am testing adding and deleting customers in my Sandbox before going live.

First, I query my database to find the customers I want to delete. The user ID in my database is the same as the customer ID in braintree. My query returns a result which is an array of users:

    if (result.length > 0) {
        for (i = 0 ; i < result.length ; i++) {
            console.log('deleting ' + result[i]['id'] + ' from braintree');
            gateway.customer.delete(result[i]['id'], function (err) {
              if (err) {
                  console.log(err);
              } else {
                  console.log('deleted customer from braintree');
              };
            }); 
        };
    };

It seems to work fine at first - I see the correct console messages. But then a few moments later it throws (doesn't just log but crashes on) this error:

10:18:53 web.1 | /.../node_modules/braintree/lib/braintree/customer.js:27
10:18:53 web.1 | if (attributes.creditCards) {
10:18:53 web.1 | ^
10:18:53 web.1 | TypeError: Cannot read property 'creditCards' of undefined
10:18:53 web.1 | at new Customer (/.../node_modules/braintree/lib/braintree/customer.js:27:19)
10:18:53 web.1 | at /.../node_modules/braintree/lib/braintree/gateway.js:121:35
10:18:53 web.1 | at /.../node_modules/braintree/lib/braintree/http.js:103:20
10:18:53 web.1 | at Parser. (/.../node_modules/braintree/node_modules/xml2js/lib/xml2js.js:134:18)
10:18:53 web.1 | at Parser.emit (events.js:107:17)
10:18:53 web.1 | at Object.saxParser.onclosetag (/.../node_modules/braintree/node_modules/xml2js/lib/xml2js.js:120:24)
10:18:53 web.1 | at emit (/.../node_modules/braintree/node_modules/xml2js/node_modules/sax/lib/sax.js:625:33)
10:18:53 web.1 | at emitNode (/.../node_modules/braintree/node_modules/xml2js/node_modules/sax/lib/sax.js:630:3)
10:18:53 web.1 | at closeTag (/.../node_modules/braintree/node_modules/xml2js/node_modules/sax/lib/sax.js:874:5)
10:18:53 web.1 | at Object.write (/.../node_modules/braintree/node_modules/xml2js/node_modules/sax/lib/sax.js:1314:29)
10:18:53 web.1 | at Parser.exports.Parser.Parser.parseString (/.../node_modules/braintree/node_modules/xml2js/lib/xml2js.js:145:29)
10:18:53 web.1 | at Parser.parseString (/.../node_modules/braintree/node_modules/xml2js/lib/xml2js.js:3:63)
10:18:53 web.1 | at IncomingMessage. (/.../node_modules/braintree/lib/braintree/http.js:102:31)
10:18:53 web.1 | at IncomingMessage.emit (events.js:129:20)
10:18:53 web.1 | at _stream_readable.js:908:16
10:18:53 web.1 | at process._tickCallback (node.js:355:11)
10:18:53 web.1 | exited with code 1

[BarthesSimpson]

I guess the problem was that I had another call to the gateway that was still running while the customer deletions were taking place, creating a race condition.