Shadowstep on Paladin bugged

''Shadowstep (10 Energy 25 yd range Instant 30 sec cooldown) - Attempts to step through the shadows and reappear behind your enemy and increases movement speed by 70% for 3 sec. The damage of your next ability is increased by 20% and the threat caused is reduced by 50%. Lasts 10 sec.''

All works fine except the part of spell which should increase the damage of my next ability for 20%, I get the buff but it doesen't increase the damage by 20% on next ability I cast, please fix it...