What is the correct encoding of a Boolean value in a SOAP message?

We have produced a web service in VB.NET against a WSDL file provided to us by our client.

In testing, our client has raised an issue complaining that the XML produced by our web service uses 'true' and 'false' for boolean values, not '1' and '0'. Furthermore, they are suggesting that our implementation is 'broken' because of this.

I was under the impression that 'true/false' and '1/0' in this context were interchangeable?

Either way, all of the XML serialisation is being handled by the .NET framework, not our code - so I assume I have little chance of changing it anyway?!

Can anyone point me to some documentation which either backs up my story or proves me wrong? If I am wrong, does anyone know how I might be able to alter this behaviour?

Thanks in advance...