Xerath's "E", Shocking Orb, does'nt stun champs that "dash" toward him.

I noticed this at first when i was battling a Xin Zhao and i "E'd" just before he dashed to me but he was'nt stun. i noticed it again when i initiated a Lee SIn as he dashed toward me and the orb flew right through him and didnt stun. Im not sure if the game was designed in such a way so some clarity on the matter would be highly appreciated. Thanks.