Error: BizTalk receives"empty" messages from WebService

Biztalk could receive "empty" messages from the Web Service, as it is deployed. Empty in this case means that the message looks like this:

<?xml version="1.0"?>
<account xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:nil="true" xmlns="http://schemas.microsoft.com/crm/2006/WebServices" />

As you can see, only the root element "account" was given (so it's not really empty).

However, the system that invokes the Web Service is definitly setting the input parameters.

This happens, if the input schema of the orchestration is changed, but the Web Service is not created and deployed afterwards (e.g. using Visual Studio's wizard). Then, the out-dated Web Service receives unexpected parameters and passes only the root node to BizTalk.