Invalid material error while PO change but not while PO deletion

Hello,

We are facing one issue at client place, where in PO change mode, when we update material with invalid material no, it gives error : The material xxxx does not exist or not activated. Message No M3305. But if we delete same line ( containing invalid material#) it is not giving error message and allows to save PO. We want the same error message even while deletion. Help..