[GAMEPLAY] Can't Cast Lux's E

[GAMEPLAY] Can't Cast Lux's E

While I was in ranked, I was farming in mid lane under tower when I used my E against a TF. All of a sudden, my E skill disappeared from the map without me casting the second part of it and from then on I couldn't cast it for the whole game. The mana cost for the skill showed as 0 mana and whenever I tried it cast it, it said "This skill cannot be cast right now". I'm not sure how to reproduce it, but I'll show how this happened.

1. In Mid Lane farming under tower against TF.
2. Lux E skill was at level 4
3. Use skill to farm the minion right in front of the tower.
4. E skill disappears without being able to cast.
5. Cannot cast E for the rest of the game.

P.S. This was during a ranked game.