السلام عليكم ورحمة الله وبركاته. For someone who reads fatihah in loud raka'at(imam reciting,. if the person reads a surah after the fatihah is this a mistake? Would sajdah-sahu be needed?

Ustadh AbdulAziz al-Haqqan

As the Imam the sunnah is that you recite another surah after the Fatihah, there is either a mistake in the question or in my understanding.