If your fund balance is zero, when regulatory charges are deducted from the empty balance, it goes into negative. This also happens if you haven’t settled the margin amount used for trading. 

Your available funds may be zero because of auto-settlement or if your Share.Market account has been inactive for a long time

Note: You can withdraw funds again only after you’ve covered the negative balance by adding that amount to your trading account. Once your balance is positive again, you can withdraw funds.