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.