How do I find out where an unhandled promise rejection occured?

Quick preface: I understand Promises, I understand resolving and rejection. This is not the question.

(node:14104) UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 1): Error: "callback" argument must be a function
(node:14104) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.

How do I find the line of code where the unhandled rejection occurred?

via mikemaccana