TypeError: Object [object Object] has no method 'apply' with Parse Cloud Code

Question

Strange Parse error:

  Input: {}
  Failed with: TypeError: Object [object Object] has no method 'apply'
    at e (Parse.js:2:6486)
    at Parse.js:2:5935
    at Array.forEach (native)
    at Object.x.each.x.forEach [as _arrayEach] (Parse.js:1:664)
    at c.extend.resolve (Parse.js:2:5886)
    at Parse.js:2:6565
    at e (Parse.js:2:6486)
    at Parse.js:2:5935
    at Array.forEach (native)
    at Object.x.each.x.forEach [as _arrayEach] (Parse.js:1:664)

As you'll notice, none of these lines refer to any code I've written.

How can I begin to debug this?