Plugin use Assign

Hi Guys,

I have used for below code with Assign Message. but I got Business Error alert once get a costume message. Please verify .

if (context.InputParameters.Contains("Target") &&
context.InputParameters["Target"] is EntityReference)
{
EntityReference targetEntity =
(EntityReference)context.InputParameters["Target"];

if (context.InputParameters.Contains("Assignee"))
{

EntityReference assigneeRef =
(EntityReference)context.InputParameters["Assignee"];

Guid modifiedById = context.InitiatingUserId;

Entity savedEntity = service.Retrieve("account", targetEntity.Id, new Microsoft.Xrm.Sdk.Query.ColumnSet(new string[] { "modifiedon", "modifiedby", "statuscode", "ownerid" }));
EntityReference Owner = savedEntity.GetAttributeValue<EntityReference>("ownerid");
if (Owner.Name != "admin")
{

throw new InvalidPluginExecutionException("This record already assigned");
}
else
{
return;
}

}

}