Issue with creating a buff that is removed after casting a spell

Hi,

I have a bit of an awkward situation with a passive ability I created. It basically grants bonus cast speed on the next spell cast after hitting with a melee weapon. I use a Trigger with 'OnLaunchesAttack' (ValidateWithAttackFilter set to True and the 'Attack Filter checks for a spell) so the buff is properly removed after one spell cast.

My issue is that 'OnLaunchesAttack' also seems to trigger off continuous attacks such as Avenging Storm, causing these effects to consume the buff even if the player hasn't cast anything yet. 

Any ideas on how to fix this ?