Is a prayer valid behind an Imam who shortens the prolongation of the letter Dhad in the word (Ad-Dalleen) in Surat Al-Fatiha?

The prayer of one who shortens the obligatory elongation (madd lazim) in the 'alif' of "ad-dallin" in Al-Fatihah is valid, because it is a grammatical error (lahn) that does not alter the meaning, whether he is an Imam or praying alone. However, a grammatical error that alters the meaning, if the person performing it knows its meaning, such as saying "Sirat alladhina an'amtu 'alayhim" (the path of those upon whom I have bestowed favor) while knowing that 'an'amtu' refers to the first-person singular, then his prayer is not valid.