dbf数据库空白字段如何批量修改为99999

Dim cn As New ADODB.Connection
Dim rs As New ADODB.Recordset
Dim cnstr As String
cnstr = "Driver={Microsoft Visual FoxPro Driver};" & _
"SourceType=DBF;" & _
"SourceDB=文件路径;" & _
"Exclusive=No"
cn.Open cnstr
cn.Execute ("update 表名 set 字段 = 99999 where isnull(字段,'')=''")