RDS template - "Engine" required even when "DBSnapshotIdentifier" is set

[robinrob]

When creating an RDS instance using a Cloudformation template, the "Engine" property should not be required when "DBSnapshotIdentifier" is set, according to the Cloudformation documentation here:
http://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-properties-rds-database-instance.html#cfn-rds-dbinstance-engine

However even when i have set "DBSnapshotIdentifier" i am getting the following error:
ValueError: Resource Engine required in type AWS::RDS::DBInstance
Which goes away when i set the "Engine" property.

Here is a minimal template demonstrating the issue:

#!/usr/bin/env python

from troposphere import Template
from troposphere.rds import DBInstance

template = Template()

rds_instance = template.add_resource(DBInstance(
    'MyDB',
    DBName='test',
    AllocatedStorage=25,
    DBInstanceClass='db.m4.large',
    DBSubnetGroupName='default',
    DBSnapshotIdentifier='id'
))

print template.to_json()

Thanks.