Calling API methods from Flex WebService issue

I'm new to Flex so I might not be doing this right, but I am having an issue with the API returning either clientID or ListID as incorrect format. I know the service works since I can hook it up in c# without issue.

Here is the setup for the flex webservice:

<mx:WebService id="myservice" wsdl="http://api.createsend.com/api/api.asmx?wsdl">
        <mx:operation name="AddSubscriber" resultFormat="object" result="method_result(event)" fault="method_fault(event)" />
        <mx:operation name="GetClients" resultFormat="object" result="method_result(event)" fault="method_fault(event)" />
</mx:WebService>

// API Calls

//This works and returns an array of ojects as intended
myservice.GetClients(API_KEY);

//This returns error 101 with event.result.Code
myservice.AddSubscriber(API_KEY, LIST_ID, "email@address.com", "User Name");


Any suggestion would be great, thanks!

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