LINQ Query Issue. where and avg is not working in LINQ

Hi Dude,
I have to get a average value with where condition in LINQ. Below i have given the code. When i use this code, i got Object reference type error. What i have to do for resolve this issue?
dim GetData=(From item in DsData.table(0).row  where item("RegDate")="2/3/2009" select item("UpdatedAmt")).Average()
Kindly advise me if any one face this issue. Thanks
 
 
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