How to set SortField to a report using Visual Basic

[gundu]

Hi , when i am setting a Field Name (String) to crystal report sortfield , it is giving and error saying that , object or block variable not set.

The following is the code

Dim lField as CRAXDRT.SortField

public function SortField(istrField as String)
set lfield = istrField
<in the above line is giving error>

.....
...
end function


What might be the cause? please help me