LINQ Query on SPListItem returning "Value does not fall within the expected range."

If you are doing LINQ Qery on SHarepoint SpListItem object and getting the following error: "Value does not fall within the expected range."
Might be this little simple (addition of .ToString()) will help you to make it work.
Original Code
List items = (from li in oList.Items.OfType()
where li["UserName"] == value
select li).ToList();

change this query to like this by adding .ToString() in li["ColName"]=="Value"
Changed Code
List items = (from li in oList.Items.OfType()
where li["Username"].ToString() == value
select li).ToList();

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