Justinian wasn't Holy Roman Emperor.
Of course he was; or else Friedrich wasn't either - the title "Holy Roman Emperor" never existed and the title "Holy Roman Empire" was invented (shortly) after Friedrich's time. Don't believe the 18th/19th century retcon about the Roman Empire "ending" in 476. It was reunited, and divided again in 800/812.
The term conventionally refers to the post-Charlemagne Western Emperors--but yes, I am aware of the problems with this, and point taken.
Yes, I was referring to the Western leaders of the Holy Roman Empire, though I think starting to consider it the Holy Roman Empire with Otto I's ascension rather than Charlemagne saves a lot of messy "what is the empire" stuff in the messy and chaotic late Carolingian era.