Getting System.TypeAccessException for DataContract

Hi,

Iam currently implementing unit test for silverlight projects and iam getting the below exception when using reference for Datacontracts getting the below exception

'System.Runtime.Serialization Version=5.0.5.0 is marked with the AllowPartiallyTrustedCallersAttribute and uses the level 2 security transparent model. Level 2 transparency causes all methods in AllowPartiallyTrustedCallers assemblies to become security transparent by default, which may be the cause of this exception'

Can u please let me know why iam getting this exception and how to overcome it ?

Thanks