Object variable not set error in lotusscript

I'm trying to get input from user and replace the user entered value in particular field. I have tired the below code but i'm getting an "Object variable not set error". Not sure about where i'm missing. Please help to resolve this error. Thanks in advance.

    Dim ws As New NotesUIWorkspace
    Dim session As New NotesSession
    Dim db As NotesDatabase
    Dim doc As NotesDocument
    Dim item As NotesItem
    Set db = session.CurrentDatabase
    rfield = Inputbox("enter name")
    If rfield = "" Then 
        Exit Sub
    End If
    nval = Inputbox("Enter the purpose ")
    Call doc.ReplaceItemValue( Cstr(rfield), "Name")
    Call doc.ReplaceItemValue(Cstr(nval),"Purpose")
    Call doc.Save(False,True)
End Sub

You have not initialised the doc variable. You can set it like this:

set doc = ws.currentdocument.document

As an aside, you should also read up on error trapping. This will help you find out which line is causing issues.