How to publish Meteor aggregation with MongoDB $lookup

I have two mongodb collections named swapBuckets and locations. I would like to aggregate a location property on the swapBucket by looking up the location by locationId. This is what the data looks like in the database.

db.swapBuckets.find().pretty();
{
    "_id" : "rcmiajXkYBDWdYmjL",
    "capacity" : "5 gallon",
    "color" : "white",
    "contentStatus" : "empty",
    "locationId" : "jNjewKekCmhwTgL9d",
}

db.locations.find().pretty();
{
    "_id" : "jNjewKekCmhwTgL9d",
    "name" : "Nourish Organic Juice",
    "number" : 1,
}

I am publishing them in my Meteor app using the tunguska:reactive-aggregate package like this...

import { ReactiveAggregate } from "meteor/tunguska:reactive-aggregate";
import { SwapBucketsCollection } from "/imports/api/SwapBuckets/collection";    
    
if (Meteor.isServer) {
  Meteor.publish("swapBucketsLocation", function () {
    ReactiveAggregate(
      this,
      SwapBucketsCollection,
      [
        {
          $lookup: {
            from: "locations",
            localField: "locationId",
            foreignField: "_id",
            as: "location",
          },
        },
      ],
      { clientCollection: "swapBucketsWithLocation" }
    );
  });
}

On the client side ui (Vue.js 2), I have this in the component...

<script>
const SwapBucketsWithLocation = new Mongo.Collection('swapBucketsWithLocation');

export default {
  meteor: {
    $subscribe: {
      swapBucketsLocation: []
    },
    swapBuckets() {
      const swapBuckets = SwapBucketsWithLocation.find({}, { sort: { number: 1 } }).fetch();
      if (swapBuckets) {
        return swapBuckets;
      }
      return [];
    },
  },
};
</script>

When I look at Meteor dev tools in Chrome, there are no entries in the SwapBucketsWithLocation collection. It is created, but it is empty. In the server console, I am getting the following error...

I20220719-10:30:42.656(-6)? Exception from sub swapBucketsLocation id ACiiCuPna7WE72GB8 TypeError: Cannot convert undefined or null to object
I20220719-10:30:42.657(-6)?     at Function.entries (<anonymous>)
I20220719-10:30:42.658(-6)?     at ReactiveAggregate (packages/tunguska:reactive-aggregate/aggregate.js:56:37)
I20220719-10:30:42.658(-6)?     at Subscription._handler (imports/api/SwapBuckets/publications.js:10:5)
I20220719-10:30:42.658(-6)?     at maybeAuditArgumentChecks (packages/ddp-server/livedata_server.js:1885:12)
I20220719-10:30:42.659(-6)?     at packages/ddp-server/livedata_server.js:1107:9
I20220719-10:30:42.659(-6)?     at Meteor.EnvironmentVariable.EVp.withValue (packages/meteor.js:1257:12)
I20220719-10:30:42.659(-6)?     at Subscription._runHandler (packages/ddp-server/livedata_server.js:1106:60)
I20220719-10:30:42.659(-6)?     at Session._startSubscription (packages/ddp-server/livedata_server.js:917:9)
I20220719-10:30:42.659(-6)?     at Session.sub (packages/ddp-server/livedata_server.js:673:12)
I20220719-10:30:42.660(-6)?     at packages/ddp-server/livedata_server.js:603:43

It seems like this is a pretty basic aggregation, but I must have something wrong here. What could it be? Any help greatly appreciated.