issues trying to use $set & $unset in Compass Aggregation builder (Compass v1.20.5 & 1.21.0-beta.4 - Ubuntu 18.04)?

Mongo Shell happily executes my pipeline but Compass doesn't like it and reports format errors.

// agg pipeline that works in shell but when imported into Compass causes problems:
    [{$unwind: {
      path: '$members'
    }}, {$unwind: {
      path: '$members.name'
    }}, {$group: {
      _id: '$members.name',
      number: {$sum: 1}
    }}, {$sort: {
      number: -1
    }}, {$limit: 10
    }, {$set: {
      name: '$_id.val'
    }}, {$unset: ['_id']
    }]

See attached screenshot showing Compass errors.

Note, when imported into Compass, no errors are initially shown but the pipeline doesn't produce results. Go into the $set or $unset stage, remove an apostrophe character (or whatever) and then re-add the same identical character and the Compass errors will then be shown.