Microsoft JET Database Engine error '80040e21'

Index or primary key cannot contain a Null value.

/practice/jostar/Categoriesadd.asp, line 97

<%@ codepage = 1256 LCID = 1065  %>
<% If Session("project1_status" <> "login" Then Response.Redirect "login.asp" %>
<%

' User levels
Const ewAllowAdd = 1
Const ewAllowDelete = 2
Const ewAllowEdit = 4
Const ewAllowView = 8
Const ewAllowList = 8
Const ewAllowSearch = 8
Const ewAllowAdmin = 16
Dim ew_SecTable(1)
ew_SecTable(0) = 15
ew_SecTable(1) = 8

' Get Current Table Security
ewCurSec = 0 ' Initialise
ewCurIdx = Session("project1_status_UserLevel"
If IsNumeric(ewCurIdx) Then
    If ewCurIdx = -1 Then ' System Administrator
        ewCurSec = 31
    ElseIf ewCurIdx > 0 and ewCurIdx <= 1 + 1 Then
        ewCurSec = ew_SecTable(ewCurIdx-1)
    End If
End If
If (ewCurSec And ewAllowadd) <> ewAllowadd Then Response.Redirect "Categorieslist.asp"
%>
<%
Response.expires = 0
Response.expiresabsolute = Now() - 1
Response.addHeader "pragma", "no-cache"
Response.addHeader "cache-control", "private"
Response.CacheControl = "no-cache"
%>
<!--#include file="db.asp"-->
<!--#include file="aspmkrfn.asp"-->
<%
Response.Buffer = True

' Get action
a = Request.Form("a"
If (a = "" Or IsNull(a)) Then
 key = Request.Querystring("key"
 If key <> "" Then
  a = "C" ' Copy record
 Else
  a = "I" ' Display blank record
 End If
End If

' Open Connection to the database
Set conn = Server.CreateObject("ADODB.Connection"
conn.Open xDb_Conn_Str
Select Case a
 Case "C": ' Get a record to display
  tkey = "'" & key & "'"
  strsql = "SELECT * FROM [Categories] WHERE [Category]=" & tkey
  Set rs = Server.CreateObject("ADODB.Recordset"
  rs.Open strsql, conn
  If rs.Eof Then
   Response.Clear
   Response.Redirect "Categorieslist.asp"
  Else
   rs.MoveFirst

   ' Get the field contents
   x_Category = rs("Category"
  End If
  rs.Close
  Set rs = Nothing
 Case "A": ' Add

  ' Get fields from form
x_Category = Request.Form("x_Category"

  ' Open record
  strsql = "SELECT * FROM [Categories] WHERE 0 = 1"
  Set rs = Server.CreateObject("ADODB.Recordset"
  rs.Open strsql, conn, 1, 2
  rs.AddNew
  tmpFld = Trim(x_Category)
  If Trim(tmpFld) & "x" = "x" Then tmpFld = Null
  srchFld = Replace(tmpFld&"","'","''"
  srchFld = Replace(srchFld,"[","[[]"
  srchFld = "'" & srchFld & "'"
  strsql = "SELECT * FROM [Categories] WHERE [Category] = " & srchFld
  Set rschk = conn.Execute(strsql)
  If Not rschk.Eof Then
    Response.Write "Duplicate value for index or primary key -- Category, value = " & tmpFld & "<br>"
    Response.Write "Press [Previous Page] key to continue!"
    Response.End
  End If
  rschk.Close
  Set rschk = Nothing
  rs("Category" = tmpFld
  rs.Update
  rs.Close
  Set rs = Nothing
  conn.Close
  Set conn = Nothing
  Response.Clear
  Response.Redirect "Categorieslist.asp"
End Select
%>