The syntax definition of $zip states that 'defaults' receives an <array expression>, but if we invoke $zip with anything other than an array literal in defaults, it shows this error:

Invalid $project :: caused by :: defaults must be an array of expressions, found string : aggregate failed

Instead, expressions that resolve to arrays should also be accepted.

How to reproduce

The code below should output [[1,"A"],[2,"B"],[3,"y"]].

db.coll.insert({'short': ['x', 'y']});
zipObj = {
    inputs: [[1, 2, 3], ["A", "B"]],
    defaults: "$short",
    useLongestLength: true
};
db.coll.aggregate([{$project: {zipped: {$zip: zipObj}}}]);