数据类型转换错误＆＃34;运行时错误3421

Question

这是我用来插入数据表的代码。如果store字段为null我有错误。有一种方法可以通过验证解决这个问题吗？

Private Sub cmdAdd_Click()
If IsNull(Me.txtstore) Then
MsgBox "Please enter Store Number"
Exit Sub
Else
'add data to the table
Dim db As Database
Dim rec As Recordset
Set db = CurrentDb
Set rec = db.OpenRecordset("Select * from Stores")
rec.AddNew
rec("Stores") = Me.txtstore
rec("Tier") = Me.cbotier
rec("Company") = Me.cbocompany
rec("Market") = Me.cbomarket
rec("Region") = Me.cboregion
rec("District Manager") = Me.cbodistrictmanger
rec("Market Manager") = Me.cbomarketmanager

rec.Update

Set rs = Nothing
Set db = Nothing
'refresh sub form
frmStoressub.Requery

MsgBox "Records updated", , "Message"
End If
End Sub