Save Session State into SQL Server in ASP.Net

hi

I used below code to save session state into sqlserver:

<sessionState        mode="SQLServer" allowCustomSqlDatabase="true"        timeout="1440" sqlConnectionString="Data Source=.;Initial Catalog=ASPState;User ID=sa;Password=2008"/>

<sessionState
       mode="SQLServer" allowCustomSqlDatabase="true"
       timeout="1440" sqlConnectionString="Data Source=.;Initial Catalog=ASPState;User ID=sa;Password=2008"/>

but when I click button to save data into session this error happen:

Unable to serialize the session state. In 'StateServer' and 'SQLServer' mode, ASP.NET will serialize the session state objects, and as a result non-serializable objects or MarshalByRef objects are not permitted. The same restriction applies if similar serialization is done by the custom session state store in 'Custom' mode.

why this happen:

Best regards

neda