Error when adding permissions to an Amazon SNS topic when using the .NET libraries

Question

When attempting to add permissions to an Amazon SNS topic using the AWS SDK for .NET/1.1.0.1 using code similar to the following:

AddPermissionRequest request = new AddPermissionRequest()
    .WithActionNames(new[] { "Publish" })
    .WithAWSAccountIds(new[] { "xxx" })
    .WithLabel("PrincipleAllowControl")
    .WithTopicArn(resourceName);
client.AddPermission(request); 

The following error message is returned:

<ErrorResponse xmlns=" http://sns.amazonaws.com/doc/2010-03-31/"&gt;
  <Error>
    <Type>Sender</Type>
    <Code>ValidationError</Code>
    <Message>2 validation errors detected: Value null at 'actionName' failed to satisfy constraint: Member must not be null; Value null at 'aWSAccountId' failed to satisfy constraint: Member must not be null</Message>
  </Error>
  <RequestId>45054159-e46b-11df-9b30-693941920fe7</RequestId>
</ErrorResponse>