Pyromantic Pact + Indigon

Hello, i'm currently trying to boost my damage using Pyromantic Pact with Indigon unique, but apparently it does not work.

The unique stats of the Indigon are :

(35–50)% increased Cost of Skills for each 200 total Mana Spent Recently
(35–50)% increased Spell damage for each 200 total Mana you have Spent Recently
Mana Recovery other than Regeneration cannot Recover Mana


So this helmet does not increase my Infernal Flame cost of spells nor does it increase the damage.

The Pyromantic Pact states :

"Gain Infernal Flame instead of spending Mana for Skill costs"

So why isn't it working ? am i missing something ?