请教一个代码计算余额引起的问题

private sub 进货价 LostFocus()

On Error GoTo Err 进货价 LostFocus

  Me![进货金额]=Me![数量]*Me![进货价]

Exit-进货价-LostFocus:

  Exit sub

Err-进货价-LostFocus:

  MsgBox Err.Description

  Resume Exit-进货价-LostFocus

End Sub