Run Time Error 3001 Invalid Argument

Access 2003 Saving Table as Access 2000. Dell Optiplex GX620 3GIG memory, 40GIG hard drive free.
Data normally has two text fields, two date fields and four other fields.
The first record has the MRN, Account_NO, Admit_date, and Discharge_Date, but the next 5-7 lines do not have these fields on the individual reocrds.

Code from DrB1309 in a previous post worked fine on a small dataset.
This error arrives on record 591950 out of 907236.
In debug mode it points to rst.update.

Another post points at possible bad data, but compact and repair is not fixing this.
If I restart the code it produces the same error message and does not process any additional records.

Code is
Sub UpdFields()
'dim LastIDSeen as string (or long, or whatever yours is)
Dim LastACCSeen, LastMRNSeen As String
Dim LastAdmSeen, LastDisSeen As Date
Dim db As DAO.Database
Dim rst As DAO.Recordset
Dim reccount As Long

Set db = CurrentDb()
Set rst = db.OpenRecordset("Import")
rst.MoveLast
reccount = rst.RecordCount
rst.MoveFirst
Do While Not rst.EOF
       If IsNull(rst.Fields("MRN")) Then
             rst.Edit
             rst.Fields("MRN") = LastMRNSeen
             rst.Fields("ACCOUNT_NO") = LastACCSeen
             rst.Fields("Admit_Date") = LastAdmSeen
             rst.Fields("Discharge_Date") = LastDisSeen
             rst.Update      
       Else
                LastMRNSeen = rst.Fields("MRN")
                LastACCSeen = rst.Fields("ACCOUNT_NO")
                LastAdmSeen = rst.Fields("Admit_Date")
                LastDisSeen = rst.Fields("Discharge_Date")
       End If
       rst.MoveNext
       reccount = reccount - 1
'       Forms!Displayform!RecordCount = reccount
 '      Forms!Displayform!TextBox1 = rst.Fields(2)
  '    ' and so on for the number of fields you want to watch flash by
   '    Forms!Displayform.Repaint
Loop
Beep
rst.Close
db.Close
End Sub