Lux E - Lucent Singularity - Uncastable

Occurred in Black Market Brawlers - unknown if this is restricted to just this game mode.

Occurred around 5 minute mark, no enemies in vicinity. As Lux cast a level 3 ranked E "lucent singularity" on enemy minions, spell travelled and dissipated causing the spell to cost 0 mana to "reactivate" being greyed out and unusable for rest of the game.