What is the meaning of this error message ?

[grd974]

I took a doing well custom indicator and added these lines of code :

private DataSeries MMAskTot;
private DataSeries MMBidTot;
Add(new Plot(Color.DarkGreen, PlotStyle.Line, "MMAskTot"));
Add(new Plot(Color.DarkMagenta, PlotStyle.Line, "MMBidTot"));
MMAskTot = new DataSeries(this);
MMBidTot = new DataSeries(this);
MMAskTot.Set(SMA(AskTot, 20));
MMBidTot.Set(SMA(BidTot, 20));

When I compiled, I got this message :

The best overloaded method match for 'NinjaTrader.Data.DataSeries.Set(double)' has some invalid arguments
Argument '1': cannot convert from 'NinjaTrader.Indicator.SMA' to 'double'
The best overloaded method match for 'NinjaTrader.Data.DataSeries.Set(double)' has some invalid arguments
Argument '1': cannot convert from 'NinjaTrader.Indicator.SMA' to 'double'

[NinjaTrader_Ray]

Issues like:

MMAskTot.Set(SMA(AskTot, 20));

should be

MMAskTot.Set(SMA(AskTot, 20)[0]);