PageMethods System.InvalidOperationException


I spend a loat of time trying to figure out how to solve this problem of returning a dataset 
from a Ajx WebMethod PageMethods Tried differnt things finally figured out its a simple 
uncommeting of the following tags or adding these lines to web,config. 

<jsonSerialization maxJsonLength="500">

 <converters>

  <add name="DataSetConverter" type="Microsoft.Web.Preview.Script.Serialization.Converters.DataSetConverter,
 Microsoft.Web.Preview, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>

   <add name="DataRowConverter" type="Microsoft.Web.Preview.Script.Serialization.Converters.DataRowConverter
, Microsoft.Web.Preview, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>


   <add name="DataTableConverter" type="Microsoft.Web.Preview.Script.Serialization.Converters.DataTableConverter, 
Microsoft.Web.Preview, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>

 </converters>

 </jsonSerialization>