RDS create db instance within a already created aurora db cluster

gistfile1.txt

$ aws rds create-db-instance --db-instance-identifier foobar-writer --db-instance-class db.r3.large --engine aurora --db-cluster-identifier foobar --availability-zone us-west-2a

Response:

{
    "DBInstance": {
        "PubliclyAccessible": true, 
        "MasterUsername": "myuser", 
        "LicenseModel": "general-public-license", 
        "VpcSecurityGroups": [
            {
                "Status": "active", 
                "VpcSecurityGroupId": "sg-8d053ae8"
            }
        ], 
        "CopyTagsToSnapshot": false, 
        "OptionGroupMemberships": [
            {
                "Status": "in-sync", 
                "OptionGroupName": "default:aurora-5-6"
            }
        ], 
        "PendingModifiedValues": {}, 
        "Engine": "aurora", 
        "MultiAZ": false, 
        "DBSecurityGroups": [], 
        "DBParameterGroups": [
            {
                "DBParameterGroupName": "default.aurora5.6", 
                "ParameterApplyStatus": "in-sync"
            }
        ], 
        "AutoMinorVersionUpgrade": true, 
        "PreferredBackupWindow": "12:29-12:59", 
        "DBSubnetGroup": {
            "Subnets": [
                {
                    "SubnetStatus": "Active", 
                    "SubnetIdentifier": "subnet-4d71da3a", 
                    "SubnetAvailabilityZone": {
                        "Name": "us-west-2a"
                    }
                }, 
                {
                    "SubnetStatus": "Active", 
                    "SubnetIdentifier": "subnet-9c5dc8f9", 
                    "SubnetAvailabilityZone": {
                        "Name": "us-west-2b"
                    }
                }, 
                {
                    "SubnetStatus": "Active", 
                    "SubnetIdentifier": "subnet-71ce0728", 
                    "SubnetAvailabilityZone": {
                        "Name": "us-west-2c"
                    }
                }
            ], 
            "DBSubnetGroupName": "default", 
            "VpcId": "vpc-c4e35fa1", 
            "DBSubnetGroupDescription": "my-subnet", 
            "SubnetGroupStatus": "Complete"
        }, 
        "ReadReplicaDBInstanceIdentifiers": [], 
        "AllocatedStorage": 1, 
        "BackupRetentionPeriod": 1, 
        "PreferredMaintenanceWindow": "thu:08:34-thu:09:04", 
        "DBInstanceStatus": "creating", 
        "EngineVersion": "5.6.10a", 
        "AvailabilityZone": "us-west-2a", 
        "DBClusterIdentifier": "foobar", 
        "StorageType": "aurora", 
        "DbiResourceId": "db-REUW64XLL3GPIFPYL4C56QT2AQ", 
        "CACertificateIdentifier": "rds-ca-2015", 
        "StorageEncrypted": false, 
        "DBInstanceClass": "db.r3.large", 
        "DbInstancePort": 0, 
        "DBInstanceIdentifier": "foobar-writer"
    }
}