The reject actually takes one parameter: that's the exception that occurred in your code that caused the promise to be rejected. So, when you call reject() the exception value is undefined, hence the "undefined" part in the error that you get.
You do not show the code that uses the promise, but I reckon it is something like this:
var promise = doSth();
promise.then(function() { doSthHere(); });
Try adding an empty failure call, like this:
promise.then(function() { doSthHere(); }, function() {});
This will prevent the error to appear.
However, I would consider calling reject only in case of an actual error, and also... having empty exception handlers isn't the best programming practice.