Penalty u/s 234c

Hello,

Short term capital gain made by sale of shares  in January. Advance tax paid in Feb. Got a notice from IT dept for payment of penalty u/s 234C. My understanding is that I need not pay penalty u/s 234c since it is STCG from sale of shares ( it was not anticipated) and advance tax is paid before March 15th. Please give your suggestions.

Thanks in advance