Mocha ignores dynamically-generated tests

Trying to run a bunch of dynamically-generated tests within a single describe(). I've copied in the example from the Mocha docs, and it works fine:

describe('add()', function() {
  var tests = [
    {args: [1, 2],       expected: 3},
    {args: [1, 2, 3],    expected: 6},
    {args: [1, 2, 3, 4], expected: 10}
  ];

  tests.forEach(function(test) {
    it('correctly adds ' + test.args.length + ' args', function() {
      var res = add.apply(null, test.args);
      assert.equal(res, test.expected);
    });
  });
});

BUT, when I try iterating over an array (or cursor) from a database call the entire describe() module is ignored - no errors, just doesn't run at all and doesn't appear in the reporter.

i.e. if I replace the tests array with

const tests = Tests.find().fetch();

the whole thing is just skipped.

What am I doing wrong? Is there a way around this?