Supercharged Slam Mana Issue

Hello when i am channelling supercharged slam, with the Constricting Command Helmet (I am counted as surrounded at all times), and the Defy 2 support gem ( Mana doesnt prevent skill use while surrounded) it is still ending the channel when my mana hits 0. If i'm not mistaken it shouldnt be doing that, and i should be able channel without and ignore the mana cost