Value does not fall within the expected range in SharePoint 2010 - sharepoint-2010

Question

I am doing below things,
On Feature Activation, I'm creating a new list with 1 sample row in it.
I'm using our internal API to create a custom list which uses FieldRef.
Sytax is like, FieldRef(string staticName, string displayName, SPFieldType, ... )
And I have created a column like
public static FieldRef SalesPerson = new FieldRef("SalesPersonName", "Person Name", SPFieldType.Text);
In my web part, I'm fetching this list's items.
SPQuery query = new SPQuery();
query.ViewFields = string.Format("
IList returnlist = null;
SPList spList = spWeb.Lists.TryGetList(ListName);
if (spList != null)
{
SPListItemCollection listitems = spList.GetItems(query);
returnlist = new List(listitems.Count); //This Gives Correct Number Of Count
foreach (SPListItem item in listitems)
{
returnlist.Add(new MyClass()
{
_SalesPersonName = item[FIELDS.SalesPerson.DisplayName].ToString();
}
}
}
And This Gives Below Error :
Server Error in '/' Application.
Value does not fall within the expected range.
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.
Exception Details: System.ArgumentException: Value does not fall within the expected range.
Source Error:
An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.