SQL Server Error : Arithmetic overflow error converting expression to data type int

After running a simple aggregate sql "select sum(imagesize) from images" is producing following error:-

Arithmetic overflow error converting expression to data type int

The table "images" is a very simple, having datatype of imagesize as bigint. The table has at least 2 million rows and should not be a problem at all. Tried number of options but no luck and finally found out the issue is actually because of the aggregate function it self. Aggregate function "sum" returns int but the totals of imagesize field is exceeding the size of int and hence was showing the error message. So the fix for this is changing the sql to return bigint as follows :-

select sum(cast(imagesize as bigint)) as 'TotalImageSize' from images

Now this worked.
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