WCF Client calling method with polymorphic array fails

WCF Client calling method with polymorphic array fails

I have a WCF service with the following (example) interface:
[ServiceContract]
[ServiceKnownType(typeof(Foo))]
[ServiceKnownType(typeof(Bar))]
[ServiceKnownType(typeof(Baz))]
public interface IMyInterface
{
[OperationContract]
void ProcessMessage(Message message);
[OperationContract]
void ProcessMessages(IEnumerable<Message> messages);
}
Foo, Bar and Baz are all a type of Message.
I can call ProcessMessage() from a WCF client with either a Foo, Bar or
Baz object and everything works fine. I can't, however, call
ProcessMessages(...) with an array (or list or any other IEnumerable)
because this will fail:
There was an error while trying to serialize parameter
http://tempuri.org/:messages. The InnerException message was 'Type
'X.X.Foo' with data contract name
'Foo:http://schemas.datacontract.org/2004/07/X.X' is not expected.
Consider using a DataContractResolver or add any types not known
statically to the list of known types - for example, by using the
KnownTypeAttribute attribute or by adding them to the list of known types
passed to DataContractSerializer.'. Please see InnerException for more
details.
When I take a look at the generated client code, reference.cs, I see:
...
[System.ServiceModel.OperationContractAttribute(Action="http://tempuri.org/IMyInterface/ProcessMessage",
ReplyAction="http://tempuri.org/IMyInterface/ProcessMessageResponse")]
[System.ServiceModel.ServiceKnownTypeAttribute(typeof(X.X.Foo))]
[System.ServiceModel.ServiceKnownTypeAttribute(typeof(X.X.Bar))]
[System.ServiceModel.ServiceKnownTypeAttribute(typeof(X.X.Baz))]
void ProcessMessage(X.X.Message message);
...
[System.ServiceModel.OperationContractAttribute(Action="http://tempuri.org/IMyInterface/ProcessMessages",
ReplyAction="http://tempuri.org/IMyInterface/ProcessMessagesResponse")]
void ProcessMessages(X.X.Message[] messages);
I notice the ServiceKnownTypeAttributes are added to ProcessMessage but
not ProcessMessages. When I manually add the same set of
ServiceKnownTypeAttributes to the ProcessMessages method and I call it
from the client with an array containing a Foo, Bar and Baz it works fine.
How do I get Visual Studio to generate these attributes on the second
method too? Is my IMyInterface wrong? Did I add the
[ServiceKnownType(typeof(...))] in the wrong place? Where did I go wrong?