计算出错

Private Sub 产品编号_AfterUpdate()
On Error GoTo Err_产品编号_LostFocus
Dim StrTemp As String
Me![产品名称] = Me![产品编号].Column(1, Me![产品编号].ListIndex)
Me![需求数量] = Me![产品编号].Column(2, Me![产品编号].ListIndex)
Me![损耗数量] = Me![产品编号].Column(3, Me![产品编号].ListIndex)

'Me![实际耗用数量] = Me![需求数量] + Me![损耗数量] 计算错误的表达式

Me![实际耗用数量] = Val(Me![需求数量]) + Val(Me![损耗数量]) '正确的表达式
Exit_产品编号_LostFocus:
    Exit Sub
Err_产品编号_LostFocus:
    MsgBox Err.Description
    Resume Exit_产品编号_LostFocus
End Sub
因为文本框数据的类型是字符型，所以使用文本框数据时应使用字符串转换成数字的函数Val