Get Value From CRM Plugins

How to I Get Value From Retrieve Method Using CRM SDK Plugins 2013

This is my Code 

Error Given key was not Present in Dicitionary 

Entity vpn = CrmServices.Retrieve("lead",((EntityReference)VPNEntity["subject"]).Id, new ColumnSet("subject"));
productGUID = ((EntityReference)VPNEntity["subject"]).Id;
if (vpn.Attributes.Contains("subject"))
{
crmvpn = vpn["subject"].ToString();
Response.Write("ali" + crmvpn);
productGUID = ((EntityReference)VPNEntity["subject"]).Id;
}