John 6:2

Greek  ἐπὶ τῶν ἀσθενούντων. On the basis of those who are weak.

He is not performing miracles onto sick people, but making signs on account of sick people. τῶν ἀσθενούντων is in the genitive. The genitive of "upon" can be translated "on the basis of," "upon the grounds of," etc, depending on a variety of factors, especially the verb, and of course having the right context. ἐπὶ with a genitive is typically used of location or place, and various relations such as "dependence upon."

See Liddell-Scott for full lexigraphical definitions and usage on ἐπὶ