Web Service Dataset With Null Or Empty Spaces.

HI,
I am using XmlDataDocuments returned by webservices and query in the reports to populate the reports.

The issue I have is if the dataset used to populate the XML document has any nulls or empty spaces in any field the whole column or row is missing in the data generated by my query in the Reports. Sometimes the whole data is not being returned to the reports.

Sample query I use

<Query>
<SoapAction>http://xxxx.webservices.org/SelectReportRegisters</SoapAction>
<Method Namespace="http://xxxx.webservices.org/" Name="SelectReportRegisters">
<Parameters>
<Parameter Name="p_registerType">
<DefaultValue>All</DefaultValue>
</Parameter>
<Parameter Name="p_registerName">
<DefaultValue>All</DefaultValue>
</Parameter>
<Parameter Name="p_asOfDate">
<DefaultValue>7-1-2009</DefaultValue>
</Parameter>
</Parameters>
</Method>
</Query>

I have tested the webservice with sample data and it works well, returns data.

I use the ISNULL() in my stored procedures to avoid nulls but with empty strings, replacing them with a default value in the managed code is expensive and slows down the reports.
Is there some feature I am missing because of which nulls and empty strings are causing this problem.

I am using Reporting services 2005.

Any pointers will be truly appreciated.