What is the issue with this pipeline?

Dear sir/madam
When I run the pipeline?
"
db.movies.aggregate([
{
$match:{“countries” : “USA”, “tomatoes.viewer.rating” : { $gte : 3} }
},
{
$project: {"_id": 0, “title”: 1, “rating”: “$tomatoes.viewer.rating”, “countries”: 1, “num_favs” : {$size: “$cast”}}
}
])
"

I got this error message:
MongoDB Enterprise Cluster0-shard-0:PRIMARY> db.movies.aggregate([
… {
… $match:{“countries” : “USA”, “tomatoes.viewer.rating” : { $gte : 3} }
… },
… {
… $project: {"_id": 0, “title”: 1, “rating”: “$tomatoes.viewer.rating”, “countries”: 1, “num_favs” : {$size: “$cast”}}
… }
… ])
uncaught exception: Error: command failed: {
“operationTime” : Timestamp(1651367725, 6),
“ok” : 0,
“errmsg” : “The argument to $size must be an array, but was of type: missing”,
“code” : 17124,
“codeName” : “Location17124”,
“$clusterTime” : {
“clusterTime” : Timestamp(1651367725, 6),
“signature” : {
“hash” : BinData(0,“zzHQpuXtBcngWjtMPZhgp13TFS8=”),
“keyId” : NumberLong(“7059403200566132740”)
}
}
} with original command request: {
“aggregate” : “movies”,
“pipeline” : [
{
“$match” : {
“countries” : “USA”,
“tomatoes.viewer.rating” : {
“$gte” : 3
}
}
},
{
“$project” : {
“_id” : 0,
“title” : 1,
“rating” : “$tomatoes.viewer.rating”,
“countries” : 1,
“num_favs” : {
“$size” : “$cast”
}
}
}
],
“cursor” : {

},
"lsid" : {
	"id" : UUID("f1ef074f-5dc9-4207-93c9-f29814cd64a8")
},
"$clusterTime" : {
	"clusterTime" : Timestamp(1651367725, 6),
	"signature" : {
		"hash" : BinData(0,"zzHQpuXtBcngWjtMPZhgp13TFS8="),
		"keyId" : NumberLong("7059403200566132740")
	}
}

} on connection: connection to cluster0-shard-00-02-jxeqq.mongodb.net:27017 : aggregate failed :
_getErrorWithCode@src/mongo/shell/utils.js:25:13
doassert@src/mongo/shell/assert.js:18:14
_assertCommandWorked@src/mongo/shell/assert.js:737:17
assert.commandWorked@src/mongo/shell/assert.js:829:16
DB.prototype._runAggregate@src/mongo/shell/db.js:281:5
DBCollection.prototype.aggregate@src/mongo/shell/collection.js:1058:12

Please advise.

Thank you.

The error message says

In your pipeline you are using $size on the field cast. This means that some documents have a field cast that is not an array. The last part of the message indicate that the but was of type: missing. So some documents are missing the field cast.

You can eliminate those documents by adding more conditions in your $match. You may also use https://www.mongodb.com/docs/manual/reference/operator/aggregation/cond/ in your computation of num_favs.

I do not think your computation of num_favs will give the correct result even once your $size:$cast issue is resolved. Double check the lab requirements.