Just curious, but is there any possibility there is a non-integer value in the tblassets.BuildNumber field of one of your assets?
Not sure how many assets you have, but a quick and dirty way could be to run the following as a new report and cast your eye over it:
Select Top 1000000 tblassets.AssetID,
tblassets.AssetName,
tblassets.BuildNumber
From tblassets
Where tblassets.BuildNumber Is Not Null