SSRS expression - If null value then return 0 instead of blank

=IIf(IsNothing(Sum(Fields!Total.Value, "AutoDeliveryCount"))=True, 0, Sum(Fields!Total.Value, "AutoDeliveryCount"))

Simply:

=Sum(Fields!Total.Value, "AutoDeliveryCount") + 0

If you want to show 0 in integer column. Write Round(Fields!yourfield.Value ,2) to display as 0.00 . If your field is returning empty.