When the Imam forgets to recite Surah Al-Fatihah

Q: I performed Zhuhr (Noon) Prayer in congregation, and when the Imam (the one who leads congregational Prayer) stood up after the first Tashahhud (a recitation in the sitting position in the second/ last unit of Prayer), I joined the Salah and recited Du`a'-ul-Istiftah (opening supplication when starting the Prayer). I then followed the Imam in Ruku` without reciting Surah Al-Fatihah when the Imam made Ruku` (bowing), but when he pronounced Taslim (salutation of peace ending the Prayer), I performed another Rak`ah (unit of Prayer) and offered Sujud-ul-Sahw (Prostration of Forgetfulness). Was what I did correct or (Part No. 6; Page No. 30

A: You did not need to make up for a Rak`ah simply because you did not recite Al-Fatihah. However, as long as you offered this Rak`ah thinking that failure to recite Al-Fatihah in a Rak`ah makes it incomplete and for that reason you made up for that Rak`ah and prostrated for forgetfulness, then there is no blame due on you because you are excused for your ignorance.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.