Issue with Bollinger bands

Hello guys, I have an issue the Bollinger bands indicator. The top, Main and Bottom dataseries of this indicator all return the same value. This is the code, I'd like your help please.

 _boll = Indicators.BollingerBands(Bars.ClosePrices, 12, 0, MovingAverageType.Simple);
            Print("boll bottom : " + _boll.Bottom.LastValue);
            Print("boll top : " + _boll.Top.LastValue);
            Print("boll main : " + _boll.Main.LastValue);

This is the output : 

boll bottom  : 0.778822500000001

boll top        : 0.778822500000001

boll main     : 0.778822500000001

Thanks in advance.