This line tries to convert what could be a BsonString type parameter into a BsonDocument and fails. The default (by documentation) acceptable input for a $merge aggregation pipeline operation is the collection name only (a string), but could be a complex object with properties like: into.

Workaround: I could put the, "into" property and make a BsonDocument as a $merge operation's value, but I only knew to do this by reading the source code after receiving an error that the BsonString could not be converted to a BsonDocument.

https://github.com/mongodb/mongo-csharp-driver/blob/9d0f35043498af39041e77df2655ef1aad3b030e/src/MongoDB.Driver/MongoCollectionImpl.cs#L809