[Gameplay] Xerath's stun didn't work

Xerath's stun went right trough Ahri in a fight. I was alone versus her and Tristana (who was attacking me only with basics). Both ahri and me casted our charm and stun pretty much at the same time, so they crossed each other, but her charm landed first (barely), then the stun lagged. Ahri would then Q and I die, either from Trist or Ahri, not sure. It only happened once that game an never before.