Sharepoint2010: ArgumentException: Value does not fall within the expected range

I have a lookup column 'usertype' in one of my list, that is lookup for another list. and in my code when i try to access this column,

item["usertype"]

i get an error: ArgumentException: Value does not fall within the expected range.

I tried with Lookup Resource Throttling in Central Administration. But still i am not able to access this field. And this field is not programmatically created.

Any idea to solve this..

Thanks in advance.